P2996 - example of `enum_to_string` via `switch-case`

[kris-jusiak]

Problem:

• P2996 doesn't support code generation yet which would be handy for the basic enum_to_string example.

Solution:

• Show a simple enum_to_string example with deducing this and nested switch which shows that it's possible, though, not ideal, and that it will be changed when code generation is supported.