~doyle/serde_bson.git

## serde_bson

Originally implemented as a workaround to the `bson` crate cloning every value it
comes across and it's looking like it shows significant improvement across the board
for serialisation (~80% improvement).

``````

There's a few pieces missing such as arrays and nested documents but they're not
too difficult to add, it's just that it's 2:38am and I've smashed this out in an
hour.

Pull requests welcome as always.