rustc: move pretty printing into its own module.

There's a lot of it, and it's a fairly well-defined/separate chunk of
code, so it might as well be separate.
rustc: remove a trait that is unnecessary after pretty move.

The type in the `impl` is now in the module with the trait.
rustc: implement a pretty mode to print ident/name's ctxt & gensyms.

`--pretty expanded,hygiene` is helpful with debugging macro issues,
since two identifiers/names can be textually the same, but different
internally (resulting in weird "undefined variable" errors).
auto merge of #16419 : huonw/rust/pretty-expanded-hygiene, r=pnkfelix

Different Identifiers and Names can have identical textual representations, but different internal representations, due to the macro hygiene machinery (syntax contexts and gensyms). This provides a way to see these internals by compiling with `--pretty expanded,hygiene`.

This is useful for debugging & hacking on macros (e.g. diagnosing https://github.com/rust-lang/rust/issues/15750/https://github.com/rust-lang/rust/issues/15962 likely would've been faster with this functionality).

E.g. 

```rust
#![feature(macro_rules)]
// minimal junk
#![no_std]

macro_rules! foo {
    ($x: ident) => { y + $x }
}

fn bar() {
    foo!(x)
}
```
```rust
#![feature(macro_rules)]
// minimal junk
#![no_std]


fn bar /* 61#0 */() { y /* 60#2 */ + x /* 58#3 */ }
```