[Feature] Supporting arbitrary map operations in GGML

[KerfuffleV2]

Proof of concept patch in #874 which adds MAP_UNARY and MAP_BINARY operations that can take a function pointer which is applied similarly to ggml_vec_add_f32 and friends.

Is this something that could potentially get included? It would enable projects or llama.cpp to support a larger range of models pretty easily because at least the simple version of an operation that GGML doesn't support could be used without having to make modifications to GGML itself.

One example is RWKV which requires several operations that GGML doesn't support.

[KerfuffleV2]

Completed by #874.