Enable arbitrary metadata through Measured + Monoid.
Add fold/iter tests for Slice.
Make Slice implement Monoid, by concatenation, and add tests for that and Slice's Measure()
Cleanup fold/iter tests everywhere, with the help of Fold_Test and Iter_Test
No need to cache the measure in a single{}
Can use the Measure() method in makeFTree, since child elements will always be Measured
Add Measured tests for finger tree components
Implement Split
Revert "Add wrapper structure, make metadata dictionary per-wrapper rather than global"

This reverts commit 4072662c2d25b93ddadd4a3c1556e5f021790e2f.
Revert "Add support for arbitary metadata."

This reverts commit e89c8a080fe2e40ddcda0915887961bee31ae5b3.
Merge branch 'nshepperd-measured' into metadata