Having go-cmp panic when using the purego build tag makes it hard to use
go-cmp for testing packages that themselves have purego fallbacks.

Since go-cmp can't implement its functionality without unsafe (the
"fallback" panics) and since environments that prohibit unsafe are much
less common these days anyway, simply remove purego code entirely.

Fixes #283.

Co-authored-by: Damien Neil <[email protected]>

GoDoc now supports hot linking to specific Go identifiers.
Use that feature to enhance the usability of cmp docs.

Since cmp lacks a direct dependency on cmpopts,
you are forced to use the fully-qualified import path
to references identifiers in the cmpopts package.

It can future improvements to the go/doc to handle this case,
either by rendering the doc with short form,
or by detecting the use of cmpopts from the unit tests as well
(since those can break circular dependencies).

This helper function makes it easier to specify that comparable types
are safe to directly compare with the == operator in Go.

The API does not use generics as it follows existing options like
cmp.AllowUnexported, cmpopts.IgnoreUnexported, or cmpopts.IgnoreTypes.

While generics provides type safety, the user experience is not as nice.
Our current API allows multiple types to be specified:
	cmpopts.EquateComparable(netip.Addr{}, netip.Prefix{})
While generics would not allow variadic arguments:
	cmpopts.EquateComparable[netip.Addr]()
	cmpopts.EquateComparable[netip.Prefix]()

Bump mininimum supported Go to 1.18 for net/netip type.
Start testing on Go 1.21.

Fixes #339