Prefer operator<<() to pretty()

It is more idiomatic, we didn't used it
in the past because Position::pretty(Move)
had a calling argument, but now we can.

As an added benefit, we avoid a lot of string
copies in the process because now we avoid
std::ostringstream ss.

No functional change.
Merge pull request #89 from official-stockfish/pull_no_pretty

Prefer operator<<() to pretty()

No functional change.