Rust demangle v0 format.

[wargio]

do not squash

This complete the implementation of the rust code by decoding the v0 rfc.

This implementation also includes the support for f16 and f128

Fix #63

[XVilka]

Changes look great!

UBSAN started to break on C++ code though:

>>> UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 LD_LIBRARY_PATH=/home/runner/work/rz-libdemangle/rz-libdemangle/build-asan/ MALLOC_PERTURB_=19 /home/runner/work/rz-libdemangle/rz-libdemangle/build-asan/test_cxx_gnu_v2
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stderr:
../src/gnu_v2/cplus-dem.c:432:9: runtime error: signed integer overflow: 555555555 * 10 cannot be represented in type 'int'
    #0 0x7fe046a543fd in consume_count ../src/gnu_v2/cplus-dem.c:432
    #1 0x7fe046a615e0 in demangle_class_name ../src/gnu_v2/cplus-dem.c:1659
    #2 0x7fe046a6fdf8 in demangle_class ../src/gnu_v2/cplus-dem.c:1718
    #3 0x7fe046a6fdf8 in demangle_signature ../src/gnu_v2/cplus-dem.c:833
    #4 0x7fe046a6fdf8 in internal_cplus_demangle ../src/gnu_v2/cplus-dem.c:640
    #5 0x7fe046a58825 in cplus_demangle_v2 ../src/gnu_v2/cplus-dem.c:594
    #6 0x7fe0469ae1bc in libdemangle_handler_cxx ../src/cxx.c:522
    #7 0x55eed49e567b in demangle_with_cxx ../test/test_cxx_gnu_v2.c:27
    #8 0x55eed49e5332 in main ../test/test_cxx_gnu_v2.c:27
    #9 0x7fe045d3a082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
    #10 0x55eed49e543d in _start (/home/runner/work/rz-libdemangle/rz-libdemangle/build-asan/test_cxx_gnu_v2+0x343d)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/gnu_v2/cplus-dem.c:432:9 in 
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Summary of Failures: