Add no_std support

[theoparis]

I've marked this as a draft since I don't know how to handle the eprintln calls yet - those need proper error handling. I did not run cargo fmt as it seems it wasn't previously ran, so it would introduce other unrelated formatting changes.

[folkertdev]

We do also use libc for malloc, free and probably some other asserted things. The println!s you could just compile out, I don't think they'd be that useful in a no_std context anyway.

But, currently, this is kind of impossible to review. For cleaning up size_t I'd suggest something more mechanical (that I can reproduce) like I suggested in #318 (comment) as a separate PR.