libiberty: Support the new ("v0") mangling scheme in rust-demangle

This is the libiberty (mainly for binutils/gdb) counterpart of
https://github.com/alexcrichton/rustc-demangle/pull/23.

Relevant links for the new Rust mangling scheme (aka "v0"):
* Rust RFC: https://github.com/rust-lang/rfcs/pull/2603
* tracking issue: https://github.com/rust-lang/rust/issues/60705
* implementation: https://github.com/rust-lang/rust/pull/57967

This implementation includes full support for UTF-8 identifiers
via punycode, so I've included a testcase for that as well.

libiberty/ChangeLog:
	* rust-demangle.c (struct rust_demangler): Add
	skipping_printing and bound_lifetime_depth fields.
	(eat): Add (v0-only).
	(parse_integer_62): Add (v0-only).
	(parse_opt_integer_62): Add (v0-only).
	(parse_disambiguator): Add (v0-only).
	(struct rust_mangled_ident): Add punycode{,_len} fields.
	(parse_ident): Support v0 identifiers.
	(print_str): Respect skipping_printing.
	(print_uint64): Add (v0-only).
	(print_uint64_hex): Add (v0-only).
	(print_ident): Respect skipping_printing,
	Support v0 identifiers.
	(print_lifetime_from_index): Add (v0-only).
	(demangle_binder): Add (v0-only).
	(demangle_path): Add (v0-only).
	(demangle_generic_arg): Add (v0-only).
	(demangle_type): Add (v0-only).
	(demangle_path_maybe_open_generics): Add (v0-only).
	(demangle_dyn_trait): Add (v0-only).
	(demangle_const): Add (v0-only).
	(demangle_const_uint): Add (v0-only).
	(basic_type): Add (v0-only).
	(rust_demangle_callback): Support v0 symbols.
	* testsuite/rust-demangle-expected: Add v0 testcases.