gccrs: Refactor the type unification code

gccrs: Refactor the type unification code

This refactors the unification systems to be a consistent interface using
switch statements and simple functions instead of the old clunky visitor
system. This is more maintainable as it is harder to miss cases when we
can take advantages of switch statements.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>
gcc/rust/ChangeLog:

	* Make-lang.in: update names
	* backend/rust-compile-expr.cc (CompileExpr::resolve_method_address):
	update to use new interface
	* typecheck/rust-coercion.cc (TypeCoercionRules::coerce_borrowed_pointer): likewise
	* typecheck/rust-hir-type-check-base.cc (TypeCheckBase::unify_site): likewise
	* typecheck/rust-tyty.cc (BaseType::destructure): likewise
	(InferType::unify): removed old unify interface
	(ErrorType::unify): likewise
	(ADTType::unify): likewise
	(TupleType::unify): likewise
	(FnType::unify): likewise
	(FnPtr::unify): likewise
	(ClosureType::unify): likewise
	(ArrayType::unify): likewise
	(SliceType::unify): likewise
	(BoolType::unify): likewise
	(IntType::unify): likewise
	(UintType::unify): likewise
	(FloatType::unify): likewise
	(USizeType::unify): likewise
	(ISizeType::unify): likewise
	(CharType::unify): likewise
	(ReferenceType::unify): likewise
	(PointerType::unify): likewise
	(ParamType::unify): likewise
	(StrType::unify): likewise
	(NeverType::unify): likewise
	(PlaceholderType::unify): likewise
	(ProjectionType::unify): likewise
	(DynamicObjectType::unify): likewise
	* typecheck/rust-tyty.h: update destructure interface
	* typecheck/rust-tyty-rules.h: Removed.
	* typecheck/rust-unify.cc: New file.
	* typecheck/rust-unify.h: New file.

gcc/testsuite/ChangeLog:

	* rust/compile/never_type_err1.rs: Moved to...
	* rust/compile/never_type1.rs: ...here. It now works