NFC compositions can involve multiple starters, such as `\u{11347}` and
`\u{11357}`. Adjust the counting iterator in the streaming fuzzer to
only count non-starters, so that it doesn't over-count.
Fixes#76.
Once the decompose iterator sees a starter, it should immediately start
returning characters from the preceeding sequence. If the input happens
to be stream-safe, it should never get more than MAX_NONSTARTERS + plus
boundary values ahead of its inner iterator.
Dan Gohman