Reduce allocation for HPACK decompression.
Merge pull request #458 from adriancole/variable-bitset

Reduce allocation for HPACK decompression
Permit status lines like 'ICY 200 OK'
Merge pull request #459 from square/jwilson_0119_icy

Permit status lines like 'ICY 200 OK'
Top-level immutable, internal HeaderEntry class to reduce duplication around header names.  Do not emit :version in http/2.
Merge pull request #460 from adriancole/header-entry

Reduce duplication around ascii header names.  Do not emit :version in http/2.