Refactor HIR with optionals, references & newtypes

Refactor HIR with optionals, references & newtypes

The HIR made heavy use of pair and other unamed types which can be
difficult to read.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc: Use FnParam getter.
	* backend/rust-compile-expr.cc (CompileExpr::visit): Likewise.
	* backend/rust-compile-intrinsic.cc: Likewise.
	* backend/rust-compile-type.cc: Likewise.
	* checks/errors/privacy/rust-privacy-reporter.cc (PrivacyReporter::visit):
	Only visit childrens if not missing.
	* checks/errors/rust-unsafe-checker.cc (UnsafeChecker::visit): Use
	a reference instead of a raw pointer.
	* hir/tree/rust-hir-expr.h: Add presence function for return
	expression.
	* hir/tree/rust-hir-item.h: Remove take_param_name.
	* hir/tree/rust-hir.h: Make mapping getter const.
	* typecheck/rust-hir-dot-operator.cc (MethodResolver::Select): Use
	getter.
	* typecheck/rust-hir-type-check-expr.cc: Likewise.
	* typecheck/rust-hir-type-check-implitem.cc: Use FnParam vector instead
	of std::pair of Pattern and BaseType.
	* typecheck/rust-hir-type-check-item.cc: Likewise.
	* typecheck/rust-hir-type-check.cc: Likewise.
	* typecheck/rust-tyty-call.cc (TypeCheckCallExpr::visit): Use getters.
	(TypeCheckMethodCallExpr::check): Likewise.
	* typecheck/rust-tyty-cmp.h: Likewise.
	* typecheck/rust-tyty.cc: Use FnParam.
	* typecheck/rust-tyty.h (class FnParam): Add FnParam to handle function
	parameters instead of handling std::pairs.
	* typecheck/rust-unify.cc (UnifyRules::expect_fndef): Use getters.
	(UnifyRules::expect_fnptr): Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>