Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.15.9 linux/amd64

What operating system and processor architecture are you using (go env)?

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/xx/.cache/go-build"
GOENV="/home/xx/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/xx/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/xx/go"
GOPRIVATE=""
GOPROXY=""
GOROOT="/usr/lib/go-1.15"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.15/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build605241201=/tmp/go-build -gno-record-gcc-switches"

What did you do?

m wondering if we could export the ctx creation to the users so that they can make some customition, i have noticed the very early issue #17 and i did it myself by adding the following code to zstd_streaming.go, but maybe it's really not a good impl.

#cgo CPPFLAGS: -DZSTD_MULTITHREAD
#cgo CFLAGS: -pthread

func (w *Writer) SetNbWorkers(n int) error {
	if w.firstError != nil {
		return w.firstError
	}
	if err := getError(int(C.ZSTD_CCtx_setParameter(w.ctx, C.ZSTD_c_nbWorkers, C.int(n)))); err != nil {
		w.firstError = err
		return err
	}
	return nil
}

What did you expect to see?

compress&decompress ctx creation & customition avaliable

What did you see instead?

ctx created by the lib internally and it's not accessable outside