Move away from symbols

Currently, DiagrammaticEquations heavily depends on using symbols for:

1. Sorts
2. Functions
3. Variables

Each of these can be transitioned to a more expressive data structure.

1. For sorts, we should use elements of an algebraic data type, as in #64. These can be serialized via intertypes.
2. For functions, we should use real Julia functions, which are overloaded for `(Sort...) -> Sort \/ Error` for type inference purposes. These can be serialized as fully-qualified module + name.
3. For variables, we should use `Path`s from Dtries. These can be serialized as vectors of symbols.

This transition is orthogonal to e-graphs/symbolicutils/decaexprs/decapodeacsets; all of these can transition away from symbols.