I don't think it makes much sense to use _BitInt for this, even if it was
in the standard, which it isn't.

You wouldn't need to parse in this function; the two byte spans would
contain the little-endian bits of the variable-length integers that make up
the integer and fraction. I don't see a compelling reason why you would
take in the integer and fraction as an integer type. You're still passing
the same data.