gccrs: introduce new types_compatable

gccrs: introduce new types_compatable

This is an initiative to begin getting rid of the can_eq interface.

Addresses #2019

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-implitem.cc (TypeCheckImplItemWithTrait::visit):
	use new interface
	* typecheck/rust-type-util.cc (types_compatable): implementation of new interface
	* typecheck/rust-type-util.h (types_compatable): prototype
	* typecheck/rust-unify.cc (UnifyRules::expect_placeholder):
	It is allow for unification against placeholders

gcc/testsuite/ChangeLog:

	* rust/compile/traits2.rs: update error message
	* rust/compile/traits3.rs: update error message

Signed-off-by: Philip Herron <herron.philip@googlemail.com>