When dealing with proc macros, there are two very different kinds of
errors:
* first, usual errors of "proc macro panicked on this particular input"
* second, the proc macro server might day if the user, eg, kills it
First kind of errors are expected and are a normal output, while the
second kind are genuine IO-errors.
For this reason, we use a curious nested result here: `Result<Result<T,
E1>, E2>` pattern, which is 100% inspired by http://sled.rs/errors.html
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.
Macro that deep clone the tokens but otherwise preserves source
locations and hygiene info is an interesting case for IDE support. Lets
have this, although we don't actively use it at the moment.
`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>.
