Consider exposing `ZSTD_f_zstd1_magicless`, possibly via `NewCtx`

[ribasushi]

As the title says - it would be great for the wrapper to somehow expose ZSTD_f_zstd1_magicless.

I store billions of compressed payloads and am stripping the first 4 bytes during storage, as they add up. My reader code looks roughly like this, with the problematic part highlighted in FastDecodeValue()

const (
	zstdMagic    = "\x28\xB5\x2F\xFD"
	zstdMagicLen = len(zstdMagic)
)

var valBuf, scratchBuf []byte
zctx := zstd.NewCtx()

for ... {
	valBuf, scratchBuf, err = FastDecodeValue(zctx, valLen, valBuf, valRaw, scratchBuf)
	...
}

func FastDecodeValue(zctx zstd.Ctx, outLen int, out, in, scratch []byte) ([]byte, []byte, error) {
	if cap(out) < int(outLen) {
		out = make([]byte, outLen)
	} else {
		out = out[:outLen]
	}

	if cap(scratch) < zstdMagicLen+len(in) {
		scratch = make([]byte, zstdMagicLen+len(in))
	}

	_, err = zctx.DecompressInto(
		out,
		append(append(scratch[:0], zstdMagic...), in...),  // <--- it would be great to remove this copying entirely
	)
	if err != nil {
		return nil, nil, xerrors.Errorf("decompression failed: %w", err)
	}

	return out, scratch, nil
}