More naming cleanup of syntax::ast types.

Continuation of https://github.com/mozilla/rust/pull/7826.

AST spanned<T> refactoring, AST type renamings:

`crate => Crate`
`local => Local`
`blk => Block`
`crate_num => CrateNum`
`crate_cfg => CrateConfig`
`field => Field`

Also, Crate, Field and Local are not wrapped in spanned<T> anymore.