Add new prettyForm binding, OPEN, which parenthesizes in Add

This is useful for things like Piecewise which need to be parenthesized even
in Add.

There is code in _print_Add that handles negatives that should probably be in
prettyForm.
Pretty print Piecewise always using parentheses

This changes the binding to OPEN, which causes it to parenthesize whenever it
is in an Add, Mul, or Pow.  This results in much easier to read expressions,
especially when a Piecewise is in an Add to the left of other terms.

This finishes the fixes of issue 3354.