Add arbitrary method to foreign_method_t conversion helper

It might not work in every case, you still are restricted
to types that set() and get() understand.

src/vm_fun.cpp

@@ -54,6 +54,10 @@ namespace wren {
 		return static_cast<int>(vm.slot_double(slot_num));
 	}
 
+	template<> std::size_t get<std::size_t> (VM& vm, int slot_num) {
+		return static_cast<std::size_t>(vm.slot_double(slot_num));
+	}
+
 	template<> std::string get<std::string> (VM& vm, int slot_num) {
 		return {vm.slot_string(slot_num)};
 	}