Use linked-hash-map to implement OrderedDocument

[zonyitoo]

The ordered::OrderedDocument is a temporary solution while the linked-hash-map crate can only be compiled with nightly. But now, linked-hash-map can be compiled with the latest stable rustc v1.7 (contain-rs/linked-hash-map#28).

So I suggest that we could migrate it to linked-hash-map.

[dariusc93]

How would this impact current use of bson?

[zonyitoo]

Currently the OrderedDocument is implemented as Vec + BTreeMap pair to maintain insertion order. Which means that we need to do a O(n) search on the Vec and store the String key twice.

Using LinkedHashMap will help to increase performance.