Add operator<<(bool) to StringBuilderImpl

Running the "Queries.IntNonIdFindOne" mongo-perf test while profiling mongod under VTune shows the StringBuilderImpl operator<<(int) as a significant hotspot. This stems from writing boolean values to StringBuilder in the read/update/delete path during query planning. As we don't implement a boolean stream handler, booleans are handled as type int, and are written inefficiently via snprintf. A boolean operator can be written that will perform the same action in a (much) more efficient manner.