Function ucdn_compose 

pub unsafe extern "C" fn ucdn_compose(
    code: *mut u32,
    a: u32,
    b: u32,
) -> c_int

Pairwise canonical composition of two codepoints. This includes
Hangul Jamo composition (see chapter 3.12 of the Unicode core
specification).

Hangul composition expects either L and V jamos, or an LV
precomposed syllable and a T jamo. This is exactly the inverse
of pairwise Hangul decomposition.

Arguments

• code filled with composition
• a first codepoint
• b second codepoint
  @return success