Introduce Attributes (#962)

* introduce Attributes for classes and methods

test/language/class/attributes/literals.wren

@@ -0,0 +1,20 @@
+// Keys must be a name, and values can be any literal value
+
+#!name = name
+#!string = "string"
+#!integer = 32
+#!number = 2.5
+#!bool = true
+class Literals {}
+
+var literalGroup = Literals.attributes.self[null]
+
+System.print(literalGroup.count)                  // expect: 5
+System.print(literalGroup["string"][0] is String) // expect: true
+System.print(literalGroup["string"][0])           // expect: string
+System.print(literalGroup["integer"][0] is Num)   // expect: true
+System.print(literalGroup["integer"][0])          // expect: 32
+System.print(literalGroup["number"][0] is Num)    // expect: true
+System.print(literalGroup["number"][0])           // expect: 2.5
+System.print(literalGroup["bool"][0] is Bool)     // expect: true
+System.print(literalGroup["bool"][0])             // expect: true