Remaining tasks before registration

[serenity4]

As requested in https://github.com/serenity4/SPIRV.jl/issues/32, here is a list of various tasks that should be tackled before registration:

• [ ] Document Mutable.
• [ ] More thoroughly document the integration between Vulkan.jl and SPIRV.jl (including but not limited to, formats).
• [ ] Go through docstrings to check that documentation is still up to date.
• [ ] Separate public and private symbols in the API reference.
• [ ] Use size-compatible unsigned integer type as a default implementation for primitive_type_to_spirv.
• [ ] Document SPIRV.MathFunctions.
• [ ] Have a documentation pass for layouts.
• [ ] Document serialize/deserialize.