Change name or symantics of expr.to_string()

[ghackebeil]

The name to_stringsuggests that the method should return a representation of itself as a string. However, this method behaves more like a pprint method because it prints the string (possibly to an output stream).

I think we should either change the name to match the behavior (e.g., something like pprint) or change the semantics of this method to something more in line with the current name.

[blnicho]

Looks like this was resolved in #272