Compiler: refactor codegen

[ysbaddaden]

Refactors Crystal::Compiler:

1. extracts #sequential_codegen, #parallel_codegen and #fork_codegen methods;
2. merges #codegen_many_units into #codegen directly;
3. stops collecting reused units: #fork_codegen now updates CompilationUnit#reused_compilation_unit? state as reported by the forked processes, and #print_codegen_stats now counts & filters the reused units.

Prerequisite for #14748 that will introduce #mt_codegen.

[ysbaddaden]

AArch64 CI is failing with "fork not implemented" 🤨

EDIT: the issue is the legacy crystal compiler in the images (1.0.0 by default 😱) that doesn't support the has_method? macro outside of methods, so we can't generate different method implementations 😮‍💨

[straight-shoota]

Actually, I prefer the version with a single def and different bodies, instead of different defs.

[ysbaddaden]

Re-created from master to fix merge conflicts with #14762 and #14763.
Added a patch to stop collecting reused units.