zstd: Add Frame header encoding and stripping

[klauspost]

Allows to remove the frame header from input and adds a function that encodes current frame header values.

New API:

// DecodeAndStrip will decode the header from the beginning of the stream
// and on success return the remaining bytes.
// This will decode the frame header and the first block header if enough bytes are provided.
// It is recommended to provide at least HeaderMaxSize bytes.
// If the frame header cannot be read an error will be returned.
// If there isn't enough input, io.ErrUnexpectedEOF is returned.
// The FirstBlock.OK will indicate if enough information was available to decode the first block header.
func (h *Header) DecodeAndStrip(in []byte) (remain []byte, err error)

// AppendTo will append the encoded header to the dst slice.
// There is no error checking performed on the header values.
func (h *Header) AppendTo(dst []byte) ([]byte, error)