make go-cmp Equal succeed

[ldemailly]

• [x] Avoid panic in go-cmp when it tries to find an Equal method
  • This one is a bit dicey as I don't know how to make it conditionally crash/not crash (crash for people who think it works, not crash for people like go-cmp for which it's only one option)

Splitting commits from #4356

[ldemailly]

'Test' is here

https://github.com/grol-io/grol/blob/v0.24.0/ast/modify_test.go#L130

panics without this fix, also

https://github.com/google/go-cmp/blob/v0.6.0/cmp/compare.go#L312-L314

in the comment hopefully makes it clear how this helps (of note, if you do have an equal method it won't be used, yet... still better than panic imo)

[aykevl]

Would it be possible to change go-cmp instead to not call MethodByName?

[ldemailly]

I don't think go-cmp are (or should be, really) concerned with adding special case for incomplete go, so it would need to be on tinygo's side (or users of both like me)

Not sure btw that it's hard to find callers without panic

maybe something with some build tag then?