zstd: add SpeedNoCompression

[dsnet]

There are situations where we must emit content as the zstd format (e.g., the protocol already negotiated ahead of time that we're operating with zstd). However, we might have some content that 1) we know won't compress well OR 2) don't want to spend CPU cycles compressing it. Could we add a SpeedNoCompression level that simply emits the appropriate zstd framing, but uses Raw_Block blocks?

This would be analogous to the existing gzip.NoCompression level.

[klauspost]

Sounds reasonable. I am currently afk, but I will be able to look at it next week!

[klauspost]

@dsnet

I am considering if it would be cleaner to have a separate wrapper, but with a common interface to the Encoder.

type Wrapper struct {}

func NewWrapper(w io.Writer, opts ...EOption) (*Wrapper, error)

func (w *Wrapper) EncodeAll(src, dst []byte) []byte

func (w *Wrapper) Reset(w io.Writer) 

func (w *Wrapper)  ResetContentSize(w io.Writer, size int64)

func (w *Wrapper)  Write(p []byte) (n int, err error) 

... etc

I think I will see how the complexity/efficiency would be around adding it to the current Encoder, as that seems to be the easiest way to use it. Just want to ensure that it only keeps minimal buffers around.

(just to update that I am looking at it, but I want it done right)