Static Extension support + small fixes

Static extension for real/custom classes. aka. the `using` keyword

Warning: UNTESTED

hscript/utils/UsingHandler.hx

@@ -0,0 +1,50 @@
+package hscript.utils;
+
+@:structInit
+class UsingEntry  {
+	public var call:Dynamic->String->Array<Dynamic>->Dynamic;
+	public var fields:Array<String>;
+
+	public function hasField(name:String) {
+		return fields.contains(name);
+	}
+}
+
+/**
+ * Special class that handles static extension function calls. 
+ * 
+ * A static extension allows pseudo-extending 
+ * existing types without modifying their source.
+ * In Haxe this is achieved by declaring a static method with a first argument 
+ * of the extending type and then bringing the defining class into context through `using`.
+ * 
+ * Example:
+ * ```haxe
+ * class IntExtender {
+ * 	static public function triple(i:Int) {
+ * 	 	return i * 3;
+ * 	}
+ * }
+ * 
+ * using IntExtender;
+ * 
+ * trace(12.triple()); // 36
+ * ```
+ * 
+ * @see https://haxe.org/manual/lf-static-extension.html
+ */
+class UsingHandler { // The ACTUAL UsingHandler.hx >:3
+	@:allow(hscript.CustomClass)
+	@:allow(hscript.CustomClassHandler)
+	public var usingEntries(default, null):Map<String, UsingEntry> = [];
+
+	public function new() {}
+
+	public function registerEntry(name:String, entry:Dynamic->String->Array<Dynamic>->Dynamic, fields:Array<String>) {
+		usingEntries.set(name, {call: entry, fields: fields});
+	}
+
+	public function entryExists(name:String):Bool {
+		return usingEntries.exists(name);
+	}
+}