Support class scoped variable lookup in interpolation lambdas (take 2)

[davepeck]

With these changes, the following code works:

def fstr(*args):
    return "".join(arg if isinstance(arg, str) else arg[0]() for arg in args)

class Example:
    food = "burrito"
    v = fstr"neato {food}"  # prior to this branch, NameError here
    
print(Example.v)  # neato burrito

This PR follows #44 but takes an approach closer to what we want. Interpolation expressions are no longer wrapped in lambdas. compiler_interpolation() and symtable_visit_expr() are both updated to support this. In addition, ste_* flags are set to allow interpolation expressions to see class scope variables.

[lysnikolaou]

@davepeck This looks great! Thanks a lot for working on these changes.

I just pushed a commit that slightly improves the code in symtable.c and brings it closer to other cases like this.

Let's merge this!

[davepeck]

slightly improves the code in symtable.c and brings it closer to other cases like this.

Ah, thanks — that's useful for me to see! Glad it wasn't too far off.