gh-87092: refactor assemble() to a number of separate functions, which do not need the compiler struct

[iritkatriel]

This PR:

1. further breaks up the assemble() function, and extracts out of it the optimize_code_unit(), assemble_code_unit(), assemble_emit() and resolve_line_numbers() steps into separate functions.
2. passes less stuff around (namely, not passing the whole compiler struct but only what is needed).
3. unit tests (_PyCompile_OptimizeCfg ) run all of optimize_code_unit(), not just optimize_cfg(), so we can test dead code removal, unused const removal, hot/cold code arrangements, etc.
4. _PyCompile_CodeGen no longer constructs a cfg.

• Issue: Split compiler into code-gen, optimizer and assembler. #87092