Module support

[bryanwjy]

C++20 module support to speed up UTL's expensive preprocessing and template processing?

Open questions/unknowns:

• Confirm that helper templates/meta functions defined in details namespaces need to be exported as well (most likely, yes)
• Do entities used by exported inline functions need to be exported? (Intuition says yes)
• Would it be useful to have PUBLIC_NAMESPACE, PRIVATE_NAMESPACE macros for internal use?
• How could it be profiled?
• How to create a modules playground to test things? Which compiler is best suited?