Add printc and exit builtin functions

tests/vm/exit_between_statements.elna

@@ -0,0 +1,5 @@
+proc main() {
+  printc('c');
+  exit();
+  printi(1234);
+}

tests/vm/print_2_statements.elna

@@ -0,0 +1,4 @@
+proc main() {
+  printi(13);
+  printi(2097150);
+}

tests/vm/print_char.elna

@@ -0,0 +1,3 @@
+proc main() {
+  printc('x');
+}

tests/vm/print_product.elna

@@ -0,0 +1,3 @@
+proc main() {
+  printi(20 * 50);
+}

tests/vm/printi_hex.elna

@@ -0,0 +1,3 @@
+proc main() {
+  printi(0x81);
+}