Add an alloc feature

[fintelia]

If there was an alloc feature in addition to the std feature then parts of the library that need allocation but not the rest of the standard library could still be used in a no_std context.

This would unblock rust-lang/rust#62502, which in turn would unblock rust-lang/rust#48331.

[alexcrichton]

I would be ok reorganizing things, but I would prefer that the decision that the libs team wants to solve rust-lang/rust#48331 be made first before the effort to reorganize things is put in.

[fintelia]

That seems reasonable. Is there something that needs to be done to get that on the libs team agenda? The discussion on that other issue seems to have stalled.

[Amanieu]

Note that this won't really solve the issue since backtrace depends on libc, which we really don't want as a dependency of alloc.

[alexcrichton]

I'm going to close this because I don't think this needs to be done at this time, and integration into libstd is a very tricky problem beyond simply shuffling imports.