feat: expose more methods to WASM builds

[Br1ght0ne]

This PR builds upon #26 and @vlopes11's comment to actually make WASM builds of fuel-crypto meaningfully usable. It requires reorganization of code in #[cfg(feature = "std")] blocks, as WASM is a no-std environment.

I imagine this PR to be unpleasant to review through diffs, so I'll list what methods were made available here when it's done™.