syntax: Shrink enum Token and enum nonterminal

`enum Token` was 192 bytes (64-bit), as pointed out by pnkfelix; the only
bloating variant being `INTERPOLATED(nonterminal)`.

Updating `enum nonterminal` to use ~ where variants included big types,
shrunk size_of(Token) to 32 bytes (64-bit).

I am unsure if the `nt_ident` variant should have an indirection, with
ast::ident being only 16 bytes (64-bit), but without this, enum Token
would be 40 bytes.