Add way to dump the AST

[emberian]

We have the ability to serialize the AST, we should use it! Probably --emit-ast would dump JSON after phase 2.

[Arnavion]

For anyone who googles rust dump ast and lands here (and doesn't look at the commit that closed this), it's rustc -Z ast-json foo.rs

[V0ldek]

This doesn't work.

rustc +nightly -Z ast-json                                                                                
error: unknown unstable option: `ast-json`

I can see an unpretty=ast-tree option, but that dumps the AST in a non-json format.

How do you get a JSON dump?