Add serialization of tokens (#3480)

- Adds tokens as a seperate proto field.
- Adds a constant table for lookup of tokens.
- Constant is a symbol table where we can store constant strings.
It is its own message, so it could potentially be used for other
objects (such as gates) in the future, but for now, it is only used
to store tokens.

Note: symbol is already used for sympy symbols, so used the
phrase Constant instead.