Rework Container.hs and improve test coverage

Since some of the functions we export from Container.hs are 1:1
identical to IntMap, we can just export the originals and remove the
wrappers. This reduces the code we need to unittest.

Furthermore, we add two simple unittest for the two non-trivial
functions that we do have in Container.hs.

And finally, we remove the 'remove' function, since it's not used, and
thus bring code coverage very close to 100%.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>