validate string interning

compilerAst/src/prog8/ast/AstToplevel.kt

@@ -282,6 +282,13 @@ class Program(val name: String,
     }
 
     fun internString(string: StringLiteralValue, dedup: Boolean): List<String> {
+        // Move a string literal into the internal, deduplicated, string pool
+        // replace it with a variable declaration that points to the entry in the pool.
+
+        if(string.parent is VarDecl) {
+            // deduplication can only be performed safely for known-const strings (=string literals OUTSIDE OF A VARDECL)!
+            throw FatalAstException("cannot intern a string literal that's part of a vardecl")
+        }
 
         fun getScopedName(string: StringLiteralValue): List<String> {
             val internedStringsBlock = modules