Native support for bytestring >=0.11

[wrengr]

bytestring-0.11.0.0 (September 2020) removed the offset parameter to the ByteString data constructor: haskell/bytestring#175 . They provide a PS pattern-synonym for backward compatibility (https://github.com/haskell/bytestring/blob/master/Changelog.md) and while CI indicates that's sufficient for now, we should surely update things to use the new internal representation directly.