LeWimbes has created PR #590: Borrow the buffer instead of owning it in bson-rust

Issue Text:
Similar to the idea in #328, `raw::Serializer` now takes a `&mut Vec<u8>`.
Because the `Vec` is borrowed, the same buffer can be reused for many serializations without reallocating.

• added *`serialize_to_buffer`*: serialize `&T` straight into a caller-supplied `Vec<u8>`.
• *`serialize_to_vec`* still exists. It just allocates a `Vec<u8>` and forwards it to `serialize_to_buffer`.

Breaking change: `raw::Serializer::new()` now expects `&mut Vec<u8>`.