Add no_std support

[Cxarli]

This PR adds the possibility to disable the std feature which makes sure the crate can run on no_std environments such as embedded devices. It also adds the alloc feature to allow using Vec even in no_std environments. Since it would be a breaking API change, it has been made opt-in.

To run the tests for this environment, you will need to run it like cargo test --no-default-features --features alloc.

Running the benchmarks locally, the performance goes slightly up.

[stusmall]

Oh this is so great. Thank you. Give me a little bit to go through and review this but this has been on my personal TODO list for a while.