gccrs: rework the HIR dump pass

gccrs: rework the HIR dump pass

Nearly complete rewrite of the HIR dump pass.

fixes #693

gcc/rust/ChangeLog:

	* hir/rust-hir-dump.cc (convert_param_kind_to_str): New.
	(convert_new_bind_type_to_str): New.
	(convert_mut_to_str): New.
	(Dump::go): New.
	(Dump::put): New.
	(Dump::begin): New.
	(Dump::end): New.
	(Dump::begin_field): New.
	(Dump::end_field): New.
	(Dump::put_field): New.
	(Dump::visit_field): New.
	(Dump::visit): Refactor.
	(Dump::visit_collection): New.
	(Dump::do_traititem): New.
	(Dump::do_vis_item): New.
	(Dump::do_functionparam): New.
	(Dump::do_pathpattern): New.
	(Dump::do_structexprstruct): New.
	(Dump::do_expr): New.
	(Dump::do_pathexpr): New.
	(Dump::do_typepathsegment): New.
	(Dump::do_typepathfunction): New.
	(Dump::do_qualifiedpathtype): New.
	(Dump::do_operatorexpr): New.
	(Dump::do_mappings): New.
	(Dump::do_inner_attrs): New.
	(Dump::do_outer_attrs): New.
	(Dump::do_baseloopexpr): New.
	(Dump::do_ifletexpr): New.
	(Dump::do_struct): New.
	(Dump::do_enumitem): New.
	(Dump::do_traitfunctiondecl): New.
	(Dump::do_externalitem): New.
	(Dump::do_namefunctionparam): New.
	(Dump::do_stmt): New.
	(Dump::do_type): New.
	(Dump::do_item): New.
	(Dump::do_tuplefield): New.
	(Dump::do_structfield): New.
	(Dump::do_genericargs): New.
	(Dump::do_maybenamedparam): New.
	* hir/rust-hir-dump.h: Refactor.
	* hir/tree/rust-hir-item.h (enum_to_str): New.
	* hir/tree/rust-hir-type.h (enum_to_str): New.
	* hir/tree/rust-hir.cc (enum_to_str): New.
	* util/rust-common.h (enum_to_str): New.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>