go/types: guarantee sorting of named type methods

[findleyr]

Certain libraries such as x/tools/go/types/objectpath rely on a well-defined sorting of methods on Named types. To enforce this definition, the objectpath package sorts, but as seen in #58668 (comment) that can be quite expensive.

We can fix that particular issue outside of go/types, but it would be nice to commit to a fixed ordering of methods in the go/types API. IIRC the ordering is currently deterministic and in source order (as defined by the input file slice), but this is not documented. We should document and preserve this behavior, or pick a different stable sort.

CC @griesemer @adonovan

[adonovan]

Strictly speaking, all that objectpath needs is that the type checker uses a deterministic ordering of Named.Methods, and that the export/import process preserves that. So either of source order and Id order would be fine, but we should at a minimum commit to determinism (in go/types) and order preservation (in gcexportdata).

[dominikh]

Related: #44195

[griesemer]

Maybe we need both, a "fast" version that doesn't guarantee an ordering, and a "stable" version that guarantees an ordering.

Right now we have sorting, but I don't think we rely on it in the type checker (and maybe we could use a map internally).