Constant substitution in CategoricalExprs

[Legotier]

Running the program

prog = compile_pgcl("""
    const c := 42
    nat x
    x := c : 1/2 + 2 : 1/2
    x := c
""")
print(prog)

gives the following output:

const c := 42;
nat x;
x := c : 1/2 + 2 : 1/2;
x := 42;

The constant in the categorical expression isn't substituted. This happens because the different subexpressions in a CategoricalExpr aren't stored as top-level attributes (sensible), but in a list of subexpressions with probabilities. This however means that the built-in walk over the expression doesn't yield these subexpressions due to the way children are discovered in mut_expr_children.

I already changed this on the brand-new branch to enable walking over the parameters of a function call. However, the problem with CategoricalExprs persists, as their subexpressions are stored in tuples, which are immutable and thus don't allow for substitution.