Deprecate `HDiv(element)` syntax?

[mscroggs]

In FEniCS/ufl#197, the ability to call the HDiv and HCurl Sobolev spaces was removed. In firedrakeproject/tsfc#302, I added a slightly hacky CallableSobolevSpace class to finat.ufl to keep the old behaviour.

Longer time, should the syntax HDiv(element) be replaced with HDivElement(element), so that CallableSobolevSpace can be removed?