Add a dependency for function2

std::function requires the function to be CopyConstructable and thus
does not allow you to capture by move, which is exactly what we need.

subprojects/function2.wrap

@@ -0,0 +1,10 @@
+[wrap-file]
+directory = function2-4.1.0
+
+source_url = https://github.com/Naios/function2/archive/4.1.0.tar.gz
+source_filename = function2-4.1.0.tar.gz
+source_hash = c3aaeaf93bf90c0f4505a18f1094b51fe28881ce202c3bf78ec4efb336c51981
+
+patch_url = file:./subprojects/function2-patch-4.1.0.tar.xz
+patch_filename = function2-patch-4.1.0.tar.xz
+patch_hash = 4b966afd862413ea1f3d96484e74401992ec958f1ee2b4cc161f3cb7c36fe7ba