We have three ways of representing binary/BSON

• mongocrypt_binary_t - public
• _mongocrypt_buffer_t - private, has conveniences
• bson_t - for working with bson

Let's be consistent with usage and behavior and document it somewhere. Also regarding const-ness, passing by pointer vs. value, and ownership semantics.

_mongocrypt_buffer_t should be used everywhere possible, and should statically initialize a bson_t when needed (perhaps it could have a bson_t member).

Consider having _mongocrypt_buffer_t data-inherit mongocrypt_binary_t, so we can return it directly by casting in public functions.

Consider where the BSON helpers for traversing/transforming should live (_mongocrypt_transform_binary_in_bson and _mongocrypt_traverse_binary_in_bson).