Add debugging aid "bmsToString(Bitmapset *bms)".

This function has no direct callers at present, but it's convenient for
manual use in a debugger, rather than having to inspect memory and do
bit-counting in your head.

In passing, get rid of useless outBitmapset() wrapper around
_outBitmapset(); let's just export the function that does the work.
Likewise for outToken().

Ashutosh Bapat, tweaked a bit by me

Discussion: <CAFjFpRdiht8e1HTVirbubr4YzaON5iZTzFJjq909y4sU8M_6eA@mail.gmail.com>