Would musl be interested in adding support for functions that convert
from / to UTF-16 and UTF-32 (assuming that char16_t is UTF-16 and
char32_t is UTF-32)?

There is the proposal N 2282, which was discussed half a year ago at the
Pittsburg meeting of WG 14; back then the comitee wanted to see a second
implementation fo these functions (the library that comes with SDCC
already has them).

Philipp