Not strictly required for libbson itself, but helps when people go
to build mongo-c-driver since it relies on it to find libbson-1.0.pc

So lets remove it, we don't want to maintain unnecessary stuff.

This adds BSON_HAVE_STDBOOL_H to bson-config.h so that we can easily
include <stdbool.h> when available. The previous check was not ever
getting used by external software because they would also have to
define HAVE_STDBOOL_H, which nobody does. And of course, this causes
inconsistencies in function calls.

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.