Consider using tagged pointers

[felixSchl]

Experimental: Use a more space efficient method to convey the type of a value

References:

• https://en.wikipedia.org/wiki/Tagged_pointer
• https://www.mikeash.com/pyblog/friday-qa-2012-07-27-lets-build-tagged-pointers.html

[felixSchl]

Irrelevant now that reference counting is implemented and purs_any_t is copy-by-value