FInAT Dual Evaluation (Round 2)

[ReubenHill]

This PR gives FInAT dual evaluation a second go (in contrast to #218 ) and sets the direction I want to take this in.

Linked PRs:

• https://github.com/firedrakeproject/firedrake/pull/2115
• https://github.com/FInAT/FInAT/pull/89