Represent names using string rather than array of parts

In most circumstances we are interested in the whole string, not
the parts split by namespace separator. As names are common, this
representation measurably improves memory usage and performance.

test/code/parser/expr/variable.test

@@ -26,9 +26,7 @@ array(
         expr: Expr_Variable(
             name: Expr_FuncCall(
                 name: Name(
-                    parts: array(
-                        0: foo
-                    )
+                    name: foo
                 )
                 args: array(
                 )