Reworked Toy_String as a union, enabled -Wpedantic

Toy now fits into the C spec.

Fixed #158

Addendum: MacOS test caught an error:

error: a function declaration without a prototype is deprecated in all versions of C

That took 3 attempts to fix correctly.

Addendum: 'No new line at the end of file' are you shitting me?

tests/cases/test_lexer.c

@@ -4,7 +4,7 @@
 #include <stdio.h>
 #include <string.h>
 
-int main() {
+int main(void) {
 	{
 		//source code sample to operate on
 		char* source = "print null;";