Duration should be considered immutable.
Store phrases as shared_ptrs.
Batch rename Source -> Phrase.
Notes about duration being nearly immutable.
Notes on future disinheritance.
Notes notes notes.
A Sequence is not a Phrase.
No more friending. const duration.
Calculate Sequence duration on call.

Sequence::getDuration() behaves like Sequence::getValue()
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.
Notes.
Updating README
Stub for Sequence decorator Phrase.
Note that Timeline is the path to Motion.
Phrase combining test restored.

Added convenience methods for generating Loops and Combines.
Add yet another phrase to combine.
Added some REQUIRE's to the combination test.