Target stable Rust

[vorot93]

This is a full list of feature flags used by tarpc.

• external_doc
• proc_macro_hygiene
• weak_counts
• non_exhaustive
• integer_atomics
• try_trait
• arbitrary_self_types
• trait_alias
• async_await
• async_closure

Now that Futures 1.0 have arrived I believe we can start dropping them one by one until tarpc works on stable Rust.

Continued from #131.

[tikue]

Probably the hardest dependencies at this point are:

1. async_await -- this can't be removed, but it will be stabilized soon (I think).
2. arbitrary_self_types -- required to implement futures by hand. I suspect it may be reaching stabilization soon.
3. proc_macro_hygiene -- required for the service! macro to generate traits etc. I have no idea how soon it will be stabilized.

Incidentally, these are also the three features that tarpc users have to enable.

[tikue]

1. external_doc -- can be replaced by copying the readme into lib.rs. It will mean the readme is subject to version skew, and we should investigate other solutions (I know some exist in the ecosystem).
2. weak_counts -- used by server::ChannelFilter and requires some investigation to remove.
3. non_exhaustive -- can replace with a doc-hidden NonExhaustive field / variant.
4. trait_alias -- used to define Transport; can be replaced by a real trait and a blanket impl.

[vorot93]

Regarding async_await: we should be able to rewrite async blocks using combinators, shouldn't we? Not a priority and decidedly not pretty but not a blocker if other nightly features get replaced before 1.38.

[tikue]

That's true, but I'd rather only do that as a last resort (i.e. if usage of all other unstable features were removed and a user was asking for it immediately). It's hugely more maintainable to use async/await. And it's only one release away :)

[tikue]

It was actually quite easy to get rid of arbitrary_self_types! I didn't realize they'd stabilized the specific usage of Pin<Self> as a receiver.

[vorot93]

async-await has hit beta, it's time to cross off the remaining feature flags asap.