this will allow passing in a bson_t that came from bson_writer_t so that
encoding packets can be transparent.

bson_json_reader_read() now expects that `bson` is uninitialized.
You should only free it if the result was 1.

Use BSON_GNUC_CONST to loader can patch in result instead of function call.
General whitespace cleanup.

fixes assert on win32.