compiler: general refactor

[aviatesk]

Separated from compiler-plugin prototyping #41632 .

[aviatesk]

I will try to do some inference benchmarking tomorrow

EDIT: I don't see performance difference by this simple benchmark:

# master
~/julia/julia master 8s
❯ ./usr/bin/julia -e '@time using Plots; @time plot(rand(10,3));'
  3.821590 seconds (7.25 M allocations: 497.022 MiB, 4.76% gc time, 0.19% compilation time)
  2.824011 seconds (3.59 M allocations: 198.979 MiB, 3.90% gc time, 55.55% compilation time)

# this PR
~/julia/julia avi/refactor 7s
❯ ./usr/bin/julia -e '@time using Plots; @time plot(rand(10,3));'
  3.554662 seconds (7.23 M allocations: 496.988 MiB, 4.32% gc time, 0.17% compilation time)
  2.856087 seconds (3.57 M allocations: 198.967 MiB, 7.94% gc time, 57.58% compilation time)

[vtjnash]

seems not unreasonable