Make CharSequence.length a function instead of property

And String.length as well.

This is done for JVM interoperability: java.lang.CharSequence is an open class
and has a function 'length()' which should be implemented in subclasses
somehow.

A minor unexpected effect of this is that String.length() is now a compile-time
constant (it wasn't such as a property because properties are not supported in
compile-time constant evaluation)

 #KT-3571 Fixed

compiler/testData/codegen/box/controlStructures/doWhile.kt

@@ -8,7 +8,7 @@ fun box(): String {
     if (y != 5) return "Fail 2 $y"
     
     var z = ""
-    do { z += z.length } while (z.length < 5)
+    do { z += z.length() } while (z.length() < 5)
     if (z != "01234") return "Fail 3 $z"
     
     return "OK"