Implement BSON Marshaler support

[ryclarke]

Adds implementation of bson.Marshaler interface to Sets.
Also addresses a locking inconsistency in UnmarshalJSON.

BSON support is important for our use cases, and should be a good improvement to this library's capabilities.

[ryclarke]

UnmarshalJSON should be a write lock - unlikely to cause issues in practice (with a newly-initialized Set), but an inconsistency nonetheless.

[deckarep]

Hi @ryclarke,

Thanks for the contribution and nice catch on the locking issue for UnmarshalJSON.

I want to make you aware of this issue if you're not already: #122
This affects the regular MarshalJSON/UnmarshalJSON api where there exists an edge case causing a panic. It will likely require a larger refactor of this code and may have implications for the BSON marshaling/unmarshaling as well.

See the issue to get more context but I have no problem in general adding support for BSON.

[ryclarke]

Thanks for the heads up - time permitting I may be able to help implement a fix for that bug in a separate PR