support decoding byte slices

[kvc0]

All of the backend infrastructure was in place to support &[u8], and some functions already take impl AsRef<[u8]>. However the main decode() and decode_header() functions require a &str. This change updates a few internal signatures and adds a _bytes() version of decode and decode_header.

When you're doing many requests per second, the cost of doing an extra utf-8 check over header payloads is significant. By supporting a &[u8] decode, users can let base64 and the crypto implementation in question handle its own bytewise validity. They already do this today in addition to the extra utf-8 scan.