consider using the `unique` package for header field names

[marten-seemann]

I'm not really familiar with the unique package, but we might be able to amortize some string allocs by not allocating new strings for header fields when decoding the header.

Of course this makes only for the header field names, not for the values in the general case. We could think about making an excepting for commonly used header field values though. A good starting point would be the QPACK static table, but we could also extend this list later.