[ENHANCEMENT] Consider making all expressions subtypes of a common supertype

[mtfishman]

For example, so that Op("X", 1) isa AbstractOperator and Op("X", 1) + Op("X", 2) isa AbstractOperator (or whatever the supertype is called, it could be Operator, OpExpr, etc.).

Alternatively, consider making all expressions runtime variants of a single type, say using algebraic data types (ADTs) as implemented in a package like Moshi.jl. That is related to the design being tested out in SymbolicArrays.jl (that package is for lazy expressions of tensors, not operators, but the design would be similar).

[mtfishman]

Note that it would still be helpful to share implementations across different types of operator-like expressions, for example state expressions as proposed in #8 (say, share the implementation of adding an argument to a sum, scaling a sum etc.). That could make use of DerivableInterfaces.jl.