Previously the s_decomposition_mappings variable would refer to other
data in s_decomposition_mappings_data. This would cause thousands of
avoidable relocations at load time.
This saves about 128kB RAM for each process which uses LibUnicode.
This fixes `combine_hangul_code_points` which would try to combine
a LVT syllable with a trailing consonant, resulting in a wrong
character.
Also added a test for this specific case.
The mappings are exposed via `Unicode::code_point_decomposition(u32)`
and `Unicode::code_point_decompositions()`, the latter being useful for
reverse searching a code point from its decomposition.
The normalization code does not make use of `Quick_Check` props (https://www.unicode.org/reports/tr44/#Decompositions_and_Normalization),
meaning no quick check optimizations.
