Support C23 _FloatN, _FloatNx, _BitInt(N), and C++23 std::bfloat16_t

[al13n321]

These productions from https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling-builtin:

  <builtin-type> ::= [...]
                 ::= DF <number> _ # ISO/IEC TS 18661 binary floating point type _FloatN (N bits), C++23 std::floatN_t
                 ::= DF <number> x # IEEE extended precision formats, C23 _FloatNx (N bits)
                 ::= DF16b # C++23 std::bfloat16_t
                 ::= DB <number> _        # C23 signed _BitInt(N)
                 ::= DB <instantiation-dependent expression> _ # C23 signed _BitInt(N)
                 ::= DU <number> _        # C23 unsigned _BitInt(N)
                 ::= DU <instantiation-dependent expression> _ # C23 unsigned _BitInt(N)

(I've seen DB8_ in the wild, everything else is just based on the 8 lines above.)

[khuey]

Thank you for the contribution. I'll do a release with this next week.

[khuey]

Released 0.4.4 on crates.io

15bbde1