[#13] Rename go module name according to NSPCC standards

- refactoring s3 gate structure
- cleanup unused code
- rename go module to `github.com/nspcc-dev/neofs-s3-gate`

closes #13

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/handler/api.go

@@ -0,0 +1,37 @@
+package handler
+
+import (
+	"errors"
+
+	"github.com/nspcc-dev/neofs-s3-gate/api"
+	"github.com/nspcc-dev/neofs-s3-gate/api/layer"
+	"go.uber.org/zap"
+)
+
+type (
+	handler struct {
+		log *zap.Logger
+		obj layer.Client
+	}
+
+	Params struct {
+		Log *zap.Logger
+		Obj layer.Client
+	}
+)
+
+var _ api.Handler = (*handler)(nil)
+
+func New(log *zap.Logger, obj layer.Client) (api.Handler, error) {
+	switch {
+	case obj == nil:
+		return nil, errors.New("empty NeoFS Object Layer")
+	case log == nil:
+		return nil, errors.New("empty logger")
+	}
+
+	return &handler{
+		log: log,
+		obj: obj,
+	}, nil
+}