refactor: `consteval` all the things!

[mpusz]

Let's try to consteval as much as we can 😉

[JohelEGP]

I'm doing that for #248.

A few bugs I've run into:

• Immediate function in requires-expression not found by ADL.
• Can't return static address from immediate function. I don't think you'll run into this one.

[mpusz]

For such cases consider introducing CONSTEVAL in hacks.h.

[mpusz]

Done in V2.