`extern crate alloc` pulled in without alloc feature

The use of extern crate alloc is unconditional, which is odd given there is an alloc feature.

My expectation would be that

everything that doesn't even depend on alloc is always built,
what needs heap allocation is only built when cfg(feature = "alloc"),
what has std dependencies is only built when cfg(feature = "std") (which entails alloc).

If that is the project's intention (and just not tested on a no-alloc system), there's a few changes I have up the pipeline but don't want to clean up to pull readiness until there is confirmation that this is how it's supposed to be:

Guard extern crate alloc by the alloc feature; doing this shows there's also one or two other things that need guarding (the utils::Context trait)
Split up the unstable feature into unstable-core and unstable (where the latter now depends on unstable-core for compatibility, and keeps pulling in std)
Make building the prelude unconditional but only include what's active (currently, the prelude is only built for std -- but the prelude is the only place through which things like FutureExt are published)
Remove from behind the cfg_alloc guard some (I won't even try to find all) things that don't really depend on alloc. The race function is what I'm actually doing this all for, as I don't want to implement it myself again. (Would be way less effort, now considering -- but doing it right may give me access to other useful traits on the long run).
Replace std with core where needed for all of that.
Figure out a way to do a meaningful test build that'd fail if anything tries to alloc. (Which is not all that simple because programs have a tendency to build even if their dependencies wouldn't as long as they don't use them).

Please let me know if the proposed is in line with the project's style, then I can hammer things out into a PR.

async-rs / async-std

`extern crate alloc` pulled in without alloc feature #952