Remove `{Pre,Post}InliningPartitioning`.

I find that these structs obfuscate the code. Removing them and just
passing the individual fields around makes the `Partition` method
signatures a little longer, but makes the data flow much clearer. E.g.

- `codegen_units` is mutable all the way through.
- `codegen_units`'s length is changed by `merge_codegen_units`, but only
  the individual elements are changed by `place_inlined_mono_items` and
  `internalize_symbols`.
- `roots`, `internalization_candidates`, and `mono_item_placements` are
  all immutable after creation, and all used by just one of the four
  methods.

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