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/print/pprust.rs

@@ -273,6 +273,7 @@ pub fn token_to_string(tok: &Token) -> String {
             token::NtIdent(e, false)    => ident_to_string(e),
             token::NtIdent(e, true)     => format!("r#{}", ident_to_string(e)),
             token::NtLifetime(e)        => ident_to_string(e),
+            token::NtLiteral(ref e)     => expr_to_string(e),
             token::NtTT(ref tree)       => tt_to_string(tree.clone()),
             token::NtArm(ref e)         => arm_to_string(e),
             token::NtImplItem(ref e)    => impl_item_to_string(e),