Use an opaque type for headers and switch representation

[gregorycollins]

Right now we use "Map ByteString ByteString" for headers -- HashMap would be much faster. The headers type should be an opaque newtype instead.

[cdsmith]

Sure, no harm in making it opaque. On the other hand, I'd be shocked to see any measurable performance difference, given that the keys are packed, basically always less than 5 machine words long, and the table will contain at most about two dozen entries.

[gregorycollins]

This is done as of 0.6.