Use TLS for big temporary structures

Given that we are already not reentrant (malloc can't be used in signal handlers and we make extensive use of it), nor can I think of reasonable scenarios where we would want that, and that we avoid recursion as much as we can (even if not explicitly a goal, it just happens to be the case), we may avoid many allocations by just using thread local storage globals (or statics if we want cleaner code) for them.
I think it's worth benchmarking to see whether it makes sense. I'm thinking mostly of the places where we would store a bitmap for mass operations.

Start by locating the bitmap storage used by mass operations and identify the relevant allocation paths. Establish benchmarks for the current allocation behavior before evaluating thread-local storage globals or statics. Done means the benchmark results support a clear decision and the affected scope is defined.