Ast spanned<T> refactoring, renaming: crate, local, blk, crate_num, crate_cfg.

`crate => Crate`
`local => Local`
`blk => Block`
`crate_num => CrateNum`
`crate_cfg => CrateConfig`

Also, Crate and Local are not wrapped in spanned<T> anymore.
De-spanned<T> and renamed ast::field (now ast::Field)