go/types: expose an API for normalized interface terms

[findleyr]

This is a placeholder for planning purposes, to be exchanged for a proper proposal at a future date.

As discussed in #60994, there are some missing go/types APIs that are currently papered over with the x/exp/typeparams package.

In particular, we should propose a go/types API that serves the purpose of the NormalTerms function -- some way to traverse a normalized representation of the terms of an interface types.

We could expose an equivalent API to NormalTerms, or do something simpler. Let's decide early in the go1.22 cycle.

CC @griesemer @adonovan @timothy-king @mdempsky

[timothy-king]

Might be helpful to go over what x/tools/go/ssa uses. The heaviest usage of NormalTerms is indirect via x/tools/internal/typeparams.CoreType. If we only release NormalTerms, CoreType will be the performance sensitive path. Other uses of NormalTerm go through the function x/tools/go/ssa.typeSetOf. The use of this is somewhat varied. FWIW all uses only pay attention to the underlying types in the list.

[gopherbot]

This issue is currently labeled as early-in-cycle for Go 1.22.
That time is now, so a friendly reminder to look at it again.