Provide explicit Bson.toString() method.

[dnadlinger]

Otherwise, opDispatch() will be invoked when trying to
writeln() a Bson object for debugging purposes, leading to
a confusing exception.

I'm not sure whether goign through Json is the best idea,
it just seemed like a convenient fix for the time being.

[s-ludwig]

That topic came up a number of times now and I always had to give an example with an explicit .toJson, so this actually makes a lot of sense. I think I was a bit too conservative here with adding methods to Bson. Going through JSON should be totally fine for now, since the output is meant purely for human inspection (no matching fromString).