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

vlib/context/value_test.v

@@ -0,0 +1,23 @@
+import context
+
+type ValueContextKey = string
+
+// This example demonstrates how a value can be passed to the context
+// and also how to retrieve it if it exists.
+fn test_with_value() {
+	f := fn (ctx context.ValueContext, key ValueContextKey) string {
+		if value := ctx.value(key) {
+			if !isnil(value) {
+				return *(&string(value))
+			}
+		}
+		return 'key not found'
+	}
+
+	key := ValueContextKey('language')
+	value := 'VAL'
+	ctx := context.with_value(context.background(), key, &value)
+
+	assert value == f(ctx, key)
+	assert 'key not found' == f(ctx, ValueContextKey('color'))
+}