[!266] Fix constraints assembly bug

Merge branch 'feature/fix-constraints-assembly-bug' into 'master'

We previously used bool() on the constraints expression to find out whether we
need to use the DIrichletConstraintsAssembler. However, UFL does the stupid
thing of evaulating

`bool(eq(x[0], 0))`

to False, because x[0] and 0 are obvioulsy not the same. Veeeery bad behaviour
for a symbolic language.

See merge request [dominic/dune-perftool!266]

  [dominic/dune-perftool!266]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/266