`extern crate alloc` pulled in without alloc feature

[chrysn]

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.

[chrysn]

(Keeping the few commits I already have in that direction on https://github.com/chrysn-pull-requests/async-std/tree/no-alloc-futures fur reference)