Fix attempting to combine Hangul Jamo 0x11a7 (#317)

Fix attempting to combine Hangul Jamo 0x11a7 (#317)

* Fix attempting to combine Hangul Jamo 0x11a7

0x11a7 is not a valid Hangul T syllable despite being equal to T_BASE.
This is because, per the Unicode spec:

  TCount is set to one more than the number of trailing consonants
  relevant to the decomposition algorithm: (0x11C2 - 0x11A8 + 1) + 1

So the first valid Hangul T syllable is 0x11a8. Also see
https://www.unicode.org/versions/Unicode17.0.0/core-spec/chapter-3/#G59434
for where the spec describes the usage of 0x11a8, not 0x11a7, during
composition.

* document that utf8proc_map simply wraps utf8proc_decompose and utf8proc_reencode (#312)

* test code refactoring (#318)

* Write regression test for #317

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Co-authored-by: Steven G. Johnson <stevenj@alum.mit.edu>