implement primary and canonical composition, and hence NFC/NFKC

implement primary and canonical composition, and hence NFC/NFKC

Read in the non-algorithmically-specified composition exclusions from
Unicode's CompositionExclusions.txt file, and generate a hash table
using the concatenated 42 bits of code points.  This is a bit of a
sucky hash-table key, particularly on 32-bit platforms; I have a plan
to reduce the key to 24 bits (using some auxiliary information in ucd)
but the advantage of getting this try in is...

... hook in NFC/NFKC into normalization tests, and check that tests
pass.