Remove Expr.__str__ dependency on prettyprint

[awf]

After #917 , Expr and prettyprint have a circular dependency.

We should make str emit s-exps without that dependency.

This will duplicate some code from prettyprinter, but much of the time we can have prettyprinter call str.

[acl33]

but much of the time we can have prettyprinter call str.

That's the part that we didn't achieve in the RLO Expression - but if we can, then sounds great :)