Update details of polymorphization working group

[davidtwco]

This PR updates the details of the polymorphization working group now that my master's project has finished.

Merge alongside rust-lang/team#398.

[lcnr]

• Initially, implement an analysis to detect when type parameters are unused in functions, closures
  and methods; and integrate this analysis into the monomorphization infrastructure so that less
  copies of functions are redundantly made (this addresses Instantiate fewer copies of a closure inside a generic function rust#46477).

Isn't this step already implemented now 😆

[davidtwco]

• Initially, implement an analysis to detect when type parameters are unused in functions, closures
  and methods; and integrate this analysis into the monomorphization infrastructure so that less
  copies of functions are redundantly made (this addresses rust-lang/rust#46477).

Isn't this step already implemented now laughing

I guess it's technically implemented, but given that we've not enabled it yet, I've left this in.