Add `positiveRange` and `isPositive`
Add `Monotonic` class

The `Monotonic` class allows querying an expression for sub-expressions in
which the expression is monotonic.
Add `Monotonic` instances relying on default implementations
Add optimization for `min` with monotonic decreasing arguments
Add monotonicDec for ShiftRU
Merge pull request #82 from Feldspar/monotonic

Perform optimizations based on the monotonicity of expressions