Now other crates don't need any 'ifdef tokio' code, since there
are wrappers that implement 'futures' right.
Technically, the 'futures' traits are in some ways less good than
the tokio ones, but we need a consistent API if we want to support
WASM someday and keep support for async_std. I'd rather hold out
hope for a future version of futures::io working like tokio than to
fix ourselves into the tokioverse forever.
This is fairly ugly and I think I'll need to mess around with the
feature configuration a while until we get something that's pleasant
to develop with. This still seems like a good idea, though, since
we _will_ need to be executor-agnostic in the end, or we'll have no
way to handle wasm or embedded environments.
Later down the road, we'll probably want to use futures::Executor or
futures::Spawn more than having global entry points in
tor_rtcompat. That would probably make our feature story simpler.
Tokio is the default now, since tokio seems to be more heavily used
for performance-critical stuff.
This patch breaks tests; the next one will fix them, albeit
questionably.
Nick Mathewson