Re-implement xtypes using the newer version of go/types

[quasilyte]

See golangci/golangci-lint#3136
See go-critic/go-critic#1253

The xtypes package is a form of a hard copy of the go/types package with some types identity rules being overwritten.
Since Go type system is changed with generics, it'll be easier to re-apply the changes on top of the newest version of stdlib go/types package.

Note that we may need to do this again in the future: there is a chance that generics will evolve over time.