Memoize split_indexed_var and get_stable_index

[KristofferC]

This makes ODEProblem creation for a somewhat large model to go from

9.323569 seconds (149.84 M allocations: 5.572 GiB, 20.05% gc time)

to

7.796214 seconds (137.43 M allocations: 5.137 GiB, 10.10% gc time)

The limit chosen is the same as for occursin_info in Symbolics.