Rounding mode support for hexadecimal float parsing, with (WIP) improvements to error handling

[quaternic]

Parsing errors are now bubbled up part of the way, but that needs some more work.

Rounding should be correct, and the Status returned by parse_any should have the correct bits set. These are used for the current (unchanged) behaviour of the surface level functions like hf64: panic on invalid inputs, or values that aren't exactly representable.