Rework towering

[mratsim]

This PR revamps towering:

• Performance:

  • towering proc handle aliasing internally, for example a.inv(a) or a.prod(a, b) will work
    without saving a as a temporary. This required an extra temporary internally but it is 1/2 the size for quadratic extensions or 1/3 for cubic extensions
  • The fancy greek letters for non-residue have been removed
  • There is no more distinction between the towering non-residue and sextic twist non-residue
    since sextic twist is chosen to allow an efficient towering
  • No more Fp2, Fp, Fp6 as return value, those are too big and lead to bad codegen.
  • pairing is now 28% faster on BLS12-381
  • before
    
  • after
    

• Organization:

  • extensions fields are now in a single file, this might help Nim with avoiding creating quintillion of proc definition when we have quadratic->cubic->quadratic Concepts building on each other: duplicated procs in C codegen nim-lang/Nim#13982. This might also help the C compiler with inline and deduplication without using --icf or -lto. This might also reduce Nim compilation times since all definitions are in a single file.
  • procs now have a clear internal and exported sections within consistent {.push inline.}/{.pop.} section