Support for no-std builds

[Kromey]

Would like the option to compile this crate in no-std environments.

Pull request #6 offers this support, although some of it may conflict with currently uncommitted work-in-progress for #2. This is a tracking issue to ensure that I circle back to it once that work is finished.

[mriise]

Just a passing comment, but depending on how things are written, It could be fairly easy to with a feature flag. Just read the PR, thats exactly what it does, but changing the flag to no-alloc instead would be more descriptive to what it does.

no-std doesn't necessarily mean no allocator, things can be written with core::alloc::Vec for anything with a global allocator but still considered no-std.

[nazar-pc]

Crate should expose alloc and std features that Serde has (or at least std if allocation is a hard requirement).