[mypyc] Provide an easier way to define IR-to-IR transforms

[JukkaL]

This makes it easy to define simple IR-to-IR transforms by subclassing IRTansform and overriding some visit methods.

Add an implementation of a simple copy propagation optimization as an example.

This will be used by the implementation of mypyc/mypyc#854, and this can also be used for various optimizations.

The IR transform preserves the identities of ops that are not modified. This means that the old IR is no longer valid after the transform, but the transform can be fast since we don't need to allocate many objects if only a small subset of ops will be modified by a transform.

[JukkaL]

cc @ilevkivskyi @msullivan

[jhance]

As mentioned before, we can implement generalized test fixture on the next IR transform we add.