Const Phrase duration. Benchmark with noop finishFn.

Finish function in benchmark proved it was very important to cache
the duration of the Sequence. By making Phrase duration immutable,
we can guarantee correct Sequence-duration without needing to do
any callbacks or cache invalidation.
More empty callbacks in benchmark test.

Should trigger the slowest possible path for measuring.
Important when considering things like always calculating vs cacheing
a particular value.