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.
It's very useful when `pub` is equivalent to "this is crate's public
API", let's enforce this!
Ideally, we should enforce it for local `cargo test`, and only during
CI, but that needs https://github.com/rust-lang/cargo/issues/5034.
