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
```

src/libsyntax/fold.rs

@@ -635,6 +635,7 @@ pub fn noop_fold_interpolated<T: Folder>(nt: token::Nonterminal, fld: &mut T)
         token::NtTy(ty) => token::NtTy(fld.fold_ty(ty)),
         token::NtIdent(ident, is_raw) => token::NtIdent(fld.fold_ident(ident), is_raw),
         token::NtLifetime(ident) => token::NtLifetime(fld.fold_ident(ident)),
+        token::NtLiteral(expr) => token::NtLiteral(fld.fold_expr(expr)),
         token::NtMeta(meta) => token::NtMeta(fld.fold_meta_item(meta)),
         token::NtPath(path) => token::NtPath(fld.fold_path(path)),
         token::NtTT(tt) => token::NtTT(fld.fold_tt(tt)),