Base.Printf: get rid of unnecessary leading underscores in names.

This was briefly the way to indicate that a name was private to a
module, but we no longer do things that way.
BigFloat printf: prevent assertion errors [#5756].

Now you get stack overflow or no method errors instead, which at
least doesn't kill the process, so that's an improvement. The real
solution is to implement decoding for BigFloat and BigInt (coming).