add context Must methods

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

flow/context.go

@@ -15,6 +15,15 @@ func FromContext(ctx context.Context) (Flow, bool) {
 	return c, ok
 }
 
+// MustContext returns Flow from context
+func MustContext(ctx context.Context) Flow {
+	f, ok := FromContext(ctx)
+	if !ok {
+		panic("missing flow")
+	}
+	return f
+}
+
 // NewContext stores Flow to context
 func NewContext(ctx context.Context, f Flow) context.Context {
 	if ctx == nil {