context: add a new context module, based on Golang's context, intended to be used in webservers (#9563)

vlib/context/empty.v

@@ -0,0 +1,37 @@
+module context
+
+import time
+
+// An EmptyContext is never canceled, has no values, and has no deadline. It is not
+// struct{}, since vars of this type must have distinct addresses.
+pub type EmptyContext = int
+
+pub fn (ctx EmptyContext) deadline() ?time.Time {
+	return none
+}
+
+pub fn (ctx EmptyContext) done() chan int {
+	ch := chan int{}
+	defer {
+		ch.close()
+	}
+	return ch
+}
+
+pub fn (ctx EmptyContext) err() string {
+	return ''
+}
+
+pub fn (ctx EmptyContext) value(key string) ?voidptr {
+	return none
+}
+
+pub fn (ctx EmptyContext) str() string {
+	if ctx == background {
+		return 'context.Background'
+	}
+	if ctx == todo {
+		return 'context.TODO'
+	}
+	return 'unknown empty Context'
+}