Add contract to master

.gitmodules

@@ -546,6 +546,10 @@
 	path = libs/callable_traits
 	url = ../callable_traits.git
 	fetchRecurseSubmodules = on-demand
+[submodule "contract"]
+	path = libs/contract
+	url = ../contract.git
+	fetchRecurseSubmodules = on-demand
 [submodule "check_build"]
 	path = tools/check_build
 	url = ../check_build.git

libs/contract

@@ -0,0 +1 @@
+Subproject commit f607e7a53faaaa108bb95f65b0b06dd1ec1ffbd7