[f24] Add conversion routines

[Zeda]

There should be conversion routines for the 24-bit floats (f24). I think maybe:

• [x] ~~f24 <== string~~
• [x] ~~f24 ==> single ("single" being this lbrary's bastardized format)~~
• [x] ~~f24 <==> binary32~~
• [ ] f24 <==> binary16
• [x] ~f24 <==> u8~
• [x] ~f24 <==> u16~
• [x] ~f24 <==> i8~
• [x] ~f24 <==> i16~