This was a bug in the `delmin` helper that is used to delete the smallest
element of the right tree. It deleted too much.

This caused erroneous calls to rotateDoubleRight instead of rotateSingleRight,
thus disbalancing the tree and possibly failing to pattern match.

This is used in the Eq instance and also makes debugging and testing easier.

This is not strictly required by copar but allows for easier testing.

This returns the size of the tree, not the actual number of elements (there can
be more elements per node) and thus should not be in the public API.

This is a requirement for its use in copar and also allows for testing, since
`shouldBe` from hspec requires those two instances.