Add a very simple test program

Not quite a unit test, it looks more like an example
but still something.

test/unit/main.cpp

@@ -0,0 +1,95 @@
+#include "wrenpp/vm_fun.hpp"
+#include "wrenpp/def_configuration.hpp"
+#include <iostream>
+#include <string_view>
+#include <map>
+#include <string>
+#include <functional>
+
+namespace {
+	const constexpr char g_script[] = ""
+R"script(class Maths {
+	foreign static mul(a,b)
+	foreign static div(a,b)
+	foreign static sum(a,b)
+	foreign static sub(a,b)
+}
+
+var my_mul = Maths.mul(Maths.mul(2, 2), Maths.mul(3, 2)) - 4
+System.print("my_mul = %(my_mul) (expected: 20)")
+
+var my_sum = Maths.sum(Maths.sum(3, 4), Maths.sum(5, -1)) + 8
+System.print("my_sum = %(my_sum) (expected: 19)")
+
+var my_sub = Maths.sub(Maths.sub(8, 5), Maths.sub(20, 19)) + 1
+System.print("my_sub = %(my_sub) (expected: 3)")
+
+)script";
+
+class MyConfig : public wren::DefConfiguration {
+	typedef std::map<std::string, wren::foreign_method_t> MethodMap;
+public:
+	wren::foreign_method_t foreign_method_fn(
+		wren::VM* vm,
+		const char* module_ptr,
+		const char* class_name_ptr,
+		bool is_static,
+		const char* signature_ptr
+	) {
+		auto key = to_map_key(module_ptr, class_name_ptr, signature_ptr);
+		auto it_found = m_foreign_methods.find(key);
+		if (m_foreign_methods.end() != it_found)
+			return it_found->second;
+		else
+			return nullptr;
+	}
+
+	void register_foreign_method (
+		std::string_view module_name,
+		std::string_view class_name,
+		std::string_view func_name,
+		wren::foreign_method_t fn
+	) {
+		m_foreign_methods[to_map_key(module_name, class_name, func_name)] = fn;
+	}
+
+private:
+	static std::string to_map_key(
+		std::string_view module_name,
+		std::string_view class_name,
+		std::string_view func_name
+	) {
+		std::string key;
+		key.reserve(module_name.size() + class_name.size() + func_name.size() + 4);
+		key.append(module_name);
+		key += "::";
+		key.append(class_name);
+		key += "::";
+		key.append(func_name);
+		return key;
+	}
+
+	MethodMap m_foreign_methods;
+};
+
+template <typename Op>
+void do_op(wren::VM& vm) {
+	using wren::get;
+	set(vm, 0, Op()(get<int>(vm, 1), get<int>(vm, 2)));
+}
+} //unnamed namespace
+
+int main() {
+	MyConfig config;
+
+	config.register_foreign_method("main", "Maths", "sum(_,_)", &do_op<std::plus<int>>);
+	config.register_foreign_method("main", "Maths", "sub(_,_)", &do_op<std::minus<int>>);
+	config.register_foreign_method("main", "Maths", "mul(_,_)", &do_op<std::multiplies<int>>);
+	config.register_foreign_method("main", "Maths", "div(_,_)", &do_op<std::divides<int>>);
+
+	wren::VM vm(&config, nullptr);
+	vm.interpret("main", g_script);
+
+	std::cout << "Dynafunc mem usage: " << vm.dynafunc_byte_size() << " bytes\n";
+	return 0;
+}

test/unit/meson.build

@@ -0,0 +1,7 @@
+exec_target = executable(meson.project_name() + '_unit',
+  'main.cpp',
+  install: false,
+  dependencies: [wrenpp_dep],
+)
+
+test(meson.project_name() + ' unit test', exec_target)