Standard decompositions are NOT always pairwise

[behdad]

Please revert 0613261. That is not true, and it breaks decomposition for U+212B ANGSTROM SIGN and possibly a few other characters. We've reverted that in HarfBuzz. Caught by HB's test-unicode.c

[grigorig]

Looks like I have some more work to do to improve UCDN's own tests. Thanks for thead heads-up!