std: Stabilize more of the `char` module

This commit performs another pass over the `std::char` module for stabilization.
Some minor cleanup is performed such as migrating documentation from libcore to
libunicode (where the `std`-facing trait resides) as well as a slight
reorganiation in libunicode itself. Otherwise, the stability modifications made
are:

* `char::from_digit` is now stable
* `CharExt::is_digit` is now stable
* `CharExt::to_digit` is now stable
* `CharExt::to_{lower,upper}case` are now stable after being modified to return
  an iterator over characters. While the implementation today has not changed
  this should allow us to implement the full set of case conversions in unicode
  where some characters can map to multiple when doing an upper or lower case
  mapping.
* `StrExt::to_{lower,upper}case` was added as unstable for a convenience of not
  having to worry about characters expanding to more characters when you just
  want the whole string to get into upper or lower case.

This is a breaking change due to the change in the signatures of the
`CharExt::to_{upper,lower}case` methods. Code can be updated to use functions
like `flat_map` or `collect` to handle the difference.

[breaking-change]

1 files changed, 4 insertions, 2 deletions

diff --git a/src/libstd/old_io/process.rs b/src/libstd/old_io/process.rs
index e02e863516a..a30dcd9d9f0 100644
--- a/src/libstd/old_io/process.rs
+++ b/src/libstd/old_io/process.rs
@@ -110,10 +110,11 @@ struct EnvKey(CString);
 #[cfg(windows)]
 impl hash::Hash for EnvKey {
     fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        use ascii::AsciiExt;
         let &EnvKey(ref x) = self;
         match str::from_utf8(x.as_bytes()) {
             Ok(s) => for ch in s.chars() {
-                (ch as u8 as char).to_lowercase().hash(state);
+                ch.to_ascii_lowercase().hash(state);
             },
             Err(..) => x.hash(state)
         }
@@ -123,6 +124,7 @@ impl hash::Hash for EnvKey {
 #[cfg(windows)]
 impl PartialEq for EnvKey {
     fn eq(&self, other: &EnvKey) -> bool {
+        use ascii::AsciiExt;
         let &EnvKey(ref x) = self;
         let &EnvKey(ref y) = other;
         match (str::from_utf8(x.as_bytes()), str::from_utf8(y.as_bytes())) {
@@ -131,7 +133,7 @@ impl PartialEq for EnvKey {
                     return false
                 } else {
                     for (xch, ych) in xs.chars().zip(ys.chars()) {
-                        if xch.to_lowercase() != ych.to_lowercase() {
+                        if xch.to_ascii_lowercase() != ych.to_ascii_lowercase() {
                             return false;
                         }
                     }