New Issue: Spike: Investigate allowing simple function calls from GPU kernels

18520, "daviditen", "Spike: Investigate allowing simple function calls from GPU kernels", "2021-10-05T16:19:24Z"

It would be useful to allow calling other simple functions from GPU kernels. As a start we could allow calling functions that are themselves as simple as the current GPU kernels e.g. no further function calls and only "fast" and "local" primitives.