`FormAssembler.assemble` not annotated for adjoint

[connorjward]

Some time ago I created persistent FormAssembler objects to remove processing overhead for repeated assemble calls. I realise now that I have failed to correctly annotate this for adjoint. To be specific, we decorate assemble with @annotate_assemble but do not do anything for FormAssembler.assemble.

I think the right fix is to emulate what we already do for solve and NonlinearVariationalSolver.solve.

This is also relevant for what I'm doing with assign (#2556) since I'm introducing a persistent Assigner class.