Macros: Add a 'literal' fragment specifier

Implements RFC 1576.

See: https://github.com/rust-lang/rfcs/blob/master/text/1576-macros-literal-matcher.md

Changes are mostly in libsyntax, docs, and tests. Feature gate is
enabled for 1.27.0.

Many thanks to Vadim Petrochenkov for following through code reviews
and suggestions.

Example:

````rust

macro_rules! test_literal {
    ($l:literal) => {
        println!("literal: {}", $l);
    };
    ($e:expr) => {
        println!("expr: {}", $e);
    };
}

fn main() {
    let a = 1;
    test_literal!(a);
    test_literal!(2);
    test_literal!(-3);
}
```

Output:

```
expr: 1
literal: 2
literal: -3
```

2 files changed, 29 insertions, 10 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 49b30c6f460..3f0df6d055b 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -115,7 +115,7 @@ macro_rules! maybe_whole_expr {
     ($p:expr) => {
         if let token::Interpolated(nt) = $p.token.clone() {
             match nt.0 {
-                token::NtExpr(ref e) => {
+                token::NtExpr(ref e) | token::NtLiteral(ref e) => {
                     $p.bump();
                     return Ok((*e).clone());
                 }
@@ -1823,7 +1823,7 @@ impl<'a> Parser<'a> {
     pub fn parse_lit_token(&mut self) -> PResult<'a, LitKind> {
         let out = match self.token {
             token::Interpolated(ref nt) => match nt.0 {
-                token::NtExpr(ref v) => match v.node {
+                token::NtExpr(ref v) | token::NtLiteral(ref v) => match v.node {
                     ExprKind::Lit(ref lit) => { lit.node.clone() }
                     _ => { return self.unexpected_last(&self.token); }
                 },
@@ -1862,7 +1862,7 @@ impl<'a> Parser<'a> {
     }
 
     /// matches '-' lit | lit (cf. ast_validation::AstValidator::check_expr_within_pat)
-    pub fn parse_pat_literal_maybe_minus(&mut self) -> PResult<'a, P<Expr>> {
+    pub fn parse_literal_maybe_minus(&mut self) -> PResult<'a, P<Expr>> {
         maybe_whole_expr!(self);
 
         let minus_lo = self.span;
@@ -2407,10 +2407,10 @@ impl<'a> Parser<'a> {
                     hi = pth.span;
                     ex = ExprKind::Path(None, pth);
                 } else {
-                    match self.parse_lit() {
-                        Ok(lit) => {
-                            hi = lit.span;
-                            ex = ExprKind::Lit(P(lit));
+                    match self.parse_literal_maybe_minus() {
+                        Ok(expr) => {
+                            hi = expr.span;
+                            ex = expr.node.clone();
                         }
                         Err(mut err) => {
                             self.cancel(&mut err);
@@ -3724,7 +3724,7 @@ impl<'a> Parser<'a> {
             let hi = self.prev_span;
             Ok(self.mk_expr(lo.to(hi), ExprKind::Path(qself, path), ThinVec::new()))
         } else {
-            self.parse_pat_literal_maybe_minus()
+            self.parse_literal_maybe_minus()
         }
     }
 
@@ -3914,7 +3914,7 @@ impl<'a> Parser<'a> {
                 }
             } else {
                 // Try to parse everything else as literal with optional minus
-                match self.parse_pat_literal_maybe_minus() {
+                match self.parse_literal_maybe_minus() {
                     Ok(begin) => {
                         if self.eat(&token::DotDotDot) {
                             let end = self.parse_pat_range_end()?;
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 938711ca1d4..6bcc1b0f026 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -280,7 +280,12 @@ impl Token {
             Lifetime(..)                      | // labeled loop
             Pound                             => true, // expression attributes
             Interpolated(ref nt) => match nt.0 {
-                NtIdent(..) | NtExpr(..) | NtBlock(..) | NtPath(..) | NtLifetime(..) => true,
+                NtLiteral(..) |
+                NtIdent(..)   |
+                NtExpr(..)    |
+                NtBlock(..)   |
+                NtPath(..)    |
+                NtLifetime(..) => true,
                 _ => false,
             },
             _ => false,
@@ -324,6 +329,18 @@ impl Token {
         }
     }
 
+    /// Returns `true` if the token is any literal, a minus (which can follow a literal,
+    /// for example a '-42', or one of the boolean idents).
+    pub fn can_begin_literal_or_bool(&self) -> bool {
+        match *self {
+            Literal(..)  => true,
+            BinOp(Minus) => true,
+            Ident(ident, false) if ident.name == keywords::True.name() => true,
+            Ident(ident, false) if ident.name == keywords::False.name() => true,
+            _            => false,
+        }
+    }
+
     /// Returns an identifier if this token is an identifier.
     pub fn ident(&self) -> Option<(ast::Ident, /* is_raw */ bool)> {
         match *self {
@@ -672,6 +689,7 @@ pub enum Nonterminal {
     NtTy(P<ast::Ty>),
     NtIdent(ast::Ident, /* is_raw */ bool),
     NtLifetime(ast::Ident),
+    NtLiteral(P<ast::Expr>),
     /// Stuff inside brackets for attributes
     NtMeta(ast::MetaItem),
     NtPath(ast::Path),
@@ -713,6 +731,7 @@ impl fmt::Debug for Nonterminal {
             NtExpr(..) => f.pad("NtExpr(..)"),
             NtTy(..) => f.pad("NtTy(..)"),
             NtIdent(..) => f.pad("NtIdent(..)"),
+            NtLiteral(..) => f.pad("NtLiteral(..)"),
             NtMeta(..) => f.pad("NtMeta(..)"),
             NtPath(..) => f.pad("NtPath(..)"),
             NtTT(..) => f.pad("NtTT(..)"),