A good list of kernels to have would str->bool kernels, similar to:
I think all but `isidentifier` make sense to have. The semantics of the Python functions seem quite reasonable to have in Arrow, but maybe others can provide feedback if this is a complete/reasonable list to have or not.
I am not sure if we need more (or less) functions, or if we want more atomic functions, e.g. test for membership in Unicode categories.