Make the simplification algorithms properly parametric

[MasonProtter]

Right now, the simplification algorithms found in src/Simplifications.jl will in theory work on any subtype of AbstractSymExpr but many of them are constructed so that they spit out a SymExpr which invalidates the purpose of making them parametric in the first place.

This isn't a high priority to fix because currently I only have one subtype of AbstractSymExpr (SymExpr) but it'd be good if I fixed this before I try implementing some other expression type that will get mysteriously simplified into a SymExpr