Auto merge of #45605 - Nashenas88:derive-newtype, r=nikomatsakis

Add derive and doc comment capabilities to newtype_index macro

This moves `RustcDecodable` and `RustcEncodable` out of the macro definition and into the macro uses. They were conflicting with `CrateNum`'s impls of `serialize::UseSpecializedEncodable` and `serialize::UseSpecializedDecodable`, and now it's not :). `CrateNum` is now defined with the `newtype_index` macro. I also added support for doc comments on constant definitions and allowed a type to remove the pub specification on the tuple param (otherwise a LOT of code would refuse to compile for `CrateNum`). I was getting dozens of errors like this if `CrateNum` was defined as `pub struct CrateNum(pub u32)`:
```
error[E0530]: match bindings cannot shadow tuple structs
   --> src/librustc/dep_graph/dep_node.rs:624:25
    |
63  | use hir::def_id::{CrateNum, DefId, DefIndex, CRATE_DEF_INDEX};
    |                   -------- a tuple struct `CrateNum` is imported here
...
624 |     [] MissingLangItems(CrateNum),
    |                         ^^^^^^^^ cannot be named the same as a tuple struct
```

I also cleaned up the formatting of the macro bodies as they were getting impossibly long. Should I go back and fix the matching rules to this style too?

I also want to see what the test results look like because `CrateNum` used to just derive `Debug`, but the `newtype_index` macro has a custom implementation. This might require further pushes.

Feel free to bikeshed on the macro language, I have no preference here.

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