bson: add bson_destroy_with_steal().

This allows, for non-read-only bson_t, to steal the underlying buffer.
In the case of inline buffers, a new buffer is allocated for the result
and returned for the convenience of the caller.

The caller must call bson_free() on the result when they are done with
the data.
bson: be a bit more defensive with stealing data.
bson: make bson_destroy_with_steal() equivalent to bson_destroy() if !steal
doc: add docs for bson_destroy_with_steal().
build: add bson_realloc_ctx() to windows ABI.
iter: cast to suseconds_t from int64_t.
added bson_new_from_buffer

a bson_new that accepts a possibly pre-alloc'd buffer controlled by
the caller and manipulated with a callback.  Useful for wrappers or
language bindings.