Rather than a "Stable" and "Nightly" ABI we instead name ABIs based on
the version of the rust compiler in which they were introduced. We place
these ABIs in a new module - `proc_macro_srv::abis` - where we also add
some mchinery to abstract over ABIs. This should make it easy to add new
ABIs at a later date as the rust compiler evolves.
`TokenStream` assumes that its subtree's delimeter is `None`, and this
should be encoded in the type system instead of having a delimiter field
that is mostly ignored.
`tt::Subtree` is just `pub delimiter: Option<Delimiter>, pub
token_trees: Vec<TokenTree>`, so a Subtree that is statically guaranteed
not to have a delimiter is just Vec<TokenTree>.
