Implement comparison on UnknownFieldSet

[LyndonGingerich]

I would like to be able to use Set operations on my generated message types, but this requires comparison. Could we implement comparison on UnknownFieldSet so F# can use structural comparison?

[LyndonGingerich]

This would probably resolve #30.

[LyndonGingerich]

I also thought of implementing custom comparison that doesn't consider UnknownFields, but then comparing two items might return 0 when equality (which does consider UnknownFields) says they're not equal.