Add no_std support

[hobofan]

Would be great if stretch would have #![no_std] support to make it work on as many platforms as possible!

Luckily all std dependencies (except for Vec) are actually from core, so this should be doable.

If you want I can prepare a PR that adds conditional no_std support via a feature flag. Sadly for now the no_std variant will only work on nightly for now, as the alloc crate (required for Vec) is unstable for now.

[emilsjolander]

I think supporting no_std would be great! How much would have to change to support this, assuming we can use alloc crate? If it's not a huge change then feel free to show me in a PR 👍