Implemented negative numbers

tests/programs/negative.q

@@ -0,0 +1,5 @@
+main() {
+	a := -32
+	b := 64
+	syscall(60, a + b)
+}

tests/programs_test.go

@@ -42,6 +42,7 @@ var programs = []struct {
 	{"loop", "", "", 0},
 	{"loop-lifetime", "", "", 0},
 	{"assert", "", "", 1},
+	{"negative", "", "", 32},
 }
 
 func TestPrograms(t *testing.T) {