Make Form::domain functions consistent

[jpdean]

As pointed out by @jorgensd, currently there are some inconsistencies in the Form::domain functions. One simply returns a span, whilst the other does some computation (to map entities from one mesh to another) and returns a vector. Since all the data computed on is immutable, we should probably compute it once in Form's constructor and store it. Then, both domain functions can return a span and we avoid unnecessary computation.