Add opt.experimental_infallible_new.

This allows a guarantee that operator new never throws.

Fix the .gitignore rules to include test/integration/cpp while we're here.

test/integration/cpp/infallible_new_false.cpp

@@ -0,0 +1,23 @@
+#include <memory>
+
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_failing_alloc) {
+	bool saw_exception = false;
+	try {
+		/* Too big of an allocation to succeed. */
+		void *volatile ptr = ::operator new((size_t)-1);
+		(void)ptr;
+	} catch (...) {
+		saw_exception = true;
+	}
+	expect_true(saw_exception, "Didn't get a failure");
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_failing_alloc);
+}
+