Symbol cache not working correctly

It seems that the symbol cache is not working correctly.
It is used to reduce the search space in terms of eclasses when performing e-matching, and should be a map of symbols -> eclasses where the symbols appear.

https://github.com/0x0f0f0f/Metatheory.jl/blob/f823eb8c733f410e10fcbb66aeeb7b71ca6c4c50/src/EGraphs/saturation.jl#L45