Add support for new UFL Constant

[miklos1]

UFL has revised its Constant node (https://bitbucket.org/fenics-project/ufl/pull-requests/116), and ufl.Constant no longer inherits from ufl.Coefficient, but directly from ufl.Terminal. This means that, given a pure UFL form, TSFC has no handler for Constants.

Firedrake is unaffected, because its own firedrake.Constant inherits directly from ufl.Coefficient and not from ufl.Constant. It is reasonable to also adopt the new UFL node in Firedrake at the same time when support is added to TSFC.