Add representation of types and expressions for initial classical expressions

[jakelishman]

What should we add?

This is tracking the work labelled "PR 1" in the initial classical-expression RFC.

Brief overview:

• write representation of Expr and Type
• test that these objects can be constructed and handle type checking correctly

[thspreetham98]

This looks very interesting. I am new to qiskit terra but I would love to work on this under someone's guidance.

[jakelishman]

I'm sorry, this (and all related issues) are not something we are soliciting external implementation contributions on.

[jakelishman]

@thspreetham98: sorry, I meant to say before that if you're interested in contributing, a good place to look would be our "good first issue" and "help wanted" labels in the issues tab up top.