Ratchet compiler performance with changes in retronym/opt/typer

[retronym]

I've measured about 2.5% compiler speedup in scala/scala@2.13.x...retronym:opt/typer

• Attribute performance improvements to commits, are they all needed?
• Polish the changes and split into PRs
• Microbenchmark to explore "Fast path for cooperative equals with exact same types."

[retronym]

Individual PRs submitted for bigger commits.

Residual: https://github.com/retronym/scala/tree/opt/typer-misc1