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?

source/toy_common.c

@@ -3,6 +3,6 @@
 //defined separately, as compilation can take several seconds, invalidating the comparisons of the given macros
 static const char* build = __DATE__ " " __TIME__ ", incomplete Toy v2.x";
 
-const char* Toy_private_version_build() {
+const char* Toy_private_version_build(void) {
 	return build;
 }