gccrs: ast: transform helper methods to visits and add methods to simplify repeated patterns

gcc/rust/ChangeLog:

	* ast/rust-ast-dump.cc (Dump::go): Use new API.
	(Dump::format_function_param): Refactor.
	(Dump::visit_items_joined_by_separator): New function.
	(Dump::emit_attrib): Refactor.
	(Dump::visit_as_line): New function.
	(Dump::visit_items_as_lines): Likewise.
	(Dump::visit_items_as_block): Likewise.
	(Dump::visit): Use new API.
	(Dump::emit_visibility): Likewise.
	(Dump::emit_indented_string): Likewise.
	(Dump::emit_generic_params): Likewise.
	(Dump::format_tuple_field): Likewise.
	(Dump::format_struct_field): Likewise.
	(Dump::format_function_common): Likewise.
	(Dump::visit_function_common): Likewise.
	* ast/rust-ast-dump.h: Declare new functions and add documentation.

Signed-off-by: Jakub Dupak <dev@jakubdupak.com>