Previously, unicode.Tokenize() would allocate a Token one-by-one, on
an as-needed basis.
This change allocates a "backing array" of Tokens, so that it goes to
the runtime object allocator much less often. It takes a heuristic
guess as to the backing array size by using the average token
(segment) length seen so far.
Results from micro-benchmark (null-firestorm, bleve-blast) seem to
give perhaps less than ~0.5 MB/second throughput improvement.
