Create header-only helper library (easel)

[axic]

We have a bunch of utilities (compiler helpers, C++20 stuff, hex strings, etc) which are re-used between intx, ethash, evmc, evmone, fizzy, hera, and now also silkworm.

It may make sense considering some header-only library with similar structure to cable, so that these projects can just include the headers either directly, via git subtree, or via a copying tool (such as cable's). The goal is to reduce maintenance burden of copying these things (and their tests) around.

A name idea is easel, which is a standing holder for paintings, but can also be derived from the ipsilon team name (ipsilon support library -> isl -> easel).