Make `assign` more permissive (again) while still respecting the adjoint

[connorjward]

Related discussion: #2743

• Modify assign.py MultiFunction to permit assigning more complex expressions (e.g. division, conj) still using numpy operations.
• Modify ad_annotate_assign to dispatch to interpolate if a "fast path" (i.e. only weighted sums) approach is not valid.
• (Maybe) Move contents of assign.py into assemble.py to reflect that assignment is just "assemble then apply".

It might make sense to make assemble into a subpackage with modules for forms, expressions and maybe interpolation.