Implement addition and subtraction

tests/expectations/print_subtraction.txt

@@ -0,0 +1 @@
+-8

tests/expectations/print_sum.txt

@@ -0,0 +1 @@
+18

tests/vm/print_subtraction.elna

@@ -0,0 +1,3 @@
+proc main() {
+  printi(5 - 13);
+}

tests/vm/print_sum.elna

@@ -0,0 +1,3 @@
+proc main() {
+  printi(5 + 13);
+}