Merge pull request #33 from robrix/reducer-control-flow

Reducer control flow
Left and right folds.
Composable Stream construction.
Stream concatenation.
Unary function composition.
Unary/binary function composition.
Binary/binary function composition.
Binary currying & uncurrying.
Ternary currying & uncurrying.
Section headings.
Remove all the uncurried composition forms.
Flatten map over streams.
Xcode does not like .x()
Fix a broken test.
Test concatenation.

This basically just copies the tests from ConcatTests; they should be
updated to test against other reducible.
Test flattenMap().
Test left and right folds.
Merge pull request #36 from robrix/flatten-map-streams

Flatten map streams
Add Either & Prelude submodules.
Add Either & Prelude to the workspace.