`NodeCountMapper` doesn't count nodes inside functions

Comparing my concatenation changes with `main` reminded me that `NodeCountMapper` doesn't currently accumulate node counts from functions into the overall count. It should be given a `map_function_definition` method that does something similar to `CallSiteCountMapper`: https://github.com/inducer/pytato/blob/78b43c197390d7aee1ec86357660d833223ddb69/pytato/analysis/__init__.py#L435-L446
I wonder if the default `map_function_definition` implementation in `CachedWalkMapper` should be disabled, to avoid bugs like this? Maybe a default implementation could be provided in a separate method, e.g. `_basic_map_function_definition`, for convenience in mappers that don't need special behavior.