Better occurrence analysis with casts
This patch addresses #20988 (closed) by refactoring the way the occurrence analyser deals with lambdas.
Previously it used collectBinders to split off a group of binders, and deal with them together. Now I deal with them one at a time in occAnalLam, which allows me to skip casts easily.
This avoidance of splitting out a list of binders has some good consequences. Less code, more efficient, and I think, more clear.