Hygienic parsing

[cbreeden]

Currently we had to dump syn::parse::token_trees since it's too slow. (It took 20 seconds to parse the symbols.rs benchmark). We use custom parsing right now, separating identifies by @ and ?, but this is not hygienic and could fail with something like:

let map: Map<A, B> = static_map! {
    Default: 0u32,
    "This @ is ? bad" => 0u32,
}