Remaining tasks before registration

[serenity4]

As requested in #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.