`alloc` feature that is still `no_std`

[jscatena88]

The use_std feature seems to be used to get access to dynamically allocated data collections (i.e. Vec). This however precludes the ability to use function such as encode_vec() in a no_std environment that has a dynamic allocator. I was able to transition to a having an alloc feature that allows access to all this while remaining no_std. The original use_std feature was left in place to avoid breaking changes for users of this crate.

Unfortunately cargo fmt created a decent bit of noise in the pull request. If that is unacceptable let me know I can make the same changes without letting cargo fmt run.

Let me know if you have any question or suggestions