Add feature-flag based support option for no_std + alloc

[ZackPierce]

This work is intended to allow bit-vec to operate in no_std environments which have access to alloc.

Existing users should not have any expected changes in use. New users interested in no_std mode (for embedded or wasm work) may opt-in by disabling a new default cargo flag, "std".

I expect to be making a similar PR to contain-rs/bit-set .

CC: @gankro @bholley

[ZackPierce]

The Travis build appears to be failing due to an unrelated build error in cargo-crusader. I have submitted a probable fix over on that repo.

Substantively, the Travis builds presently show successful cargo test runs.

[ZackPierce]

Re-attempted the travis build by closing and reopening the PR, with success!

[Gankra]

CC @alexcrichton is this the expected way to support no_std?

[alexcrichton]

I'd recommend unconditionally being flagged as #![no_std] and then whe it's enabled you explicitly extern crate std and import items from std

[ZackPierce]

Thanks for the tip, @alexcrichton , I'll switch to that approach.

For the prior commit, I was hesitant to do an unconditional no_std on account of the reliance on alloc (for Vec), but I'm happy to oblige your recommendation. New commit coming later today.

[ZackPierce]

@gankro Updated per Alex's suggestion.

[Gankra]

LGTM, thanks!