Add the ability to work in no-std (optionally with alloc)

[Dr-Emann]

By default, the std feature is enabled, but with disable-default-features, it can now be either no-std/no-alloc.

This did involve a breaking change with the serialization traits, to allow no-alloc serialization.

Serialize/Deserialize traits were sealed, and most functions were hidden

Per updated guidance, it is recommended to commit the Cargo.lock, even for libraries, so this removes it from the gitignore, and adds it.

Fixes #131