Cache output of numba_funcify where possible

[aseyboldt]

We could add cache_by_op or cache_by_op_and_sig decorators for numba_funcitfy functions, that internally use a LRU cache to keep track of previous njit functions. Each funcify function can then choose to reuse results in a way that is appropriate.
Most funcify functions only use the op, and don't even look at the node at all.
See also #179 for context.