Forbid appending unsigned integral types to BSONObjBuilders

The BSONObjBuilder has overloads that allow appending unsigned integral types (notably size_t). This doesn't have good semantics, since the underlying BSON specification doesn't offer unsigned types.

We should remove these overloads, and require that only signed types be able to be directly appended. This will force call sites to make choices, as they probably should be doing.