replace form_data.signature by form.signature
foo.form_data() -> compute_form_data(foo)
make compatible with upstream UFL; need to optimize
remove one instance of compute_form_data; tidy up
Don't compute form data if we had kernels stashed on the form
Use form.signature rather than compute_form_signature
Merge pull request #327 from firedrakeproject/upstream-compatibility

* upstream-compatibility:
  Use form.signature rather than compute_form_signature
  Don't compute form data if we had kernels stashed on the form
  remove one instance of compute_form_data; tidy up
  make compatible with upstream UFL; need to optimize
  foo.form_data() -> compute_form_data(foo)
  replace form_data.signature by form.signature