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

idea/testData/checker/regression/Jet121.kt

@@ -1,13 +1,14 @@
 package jet121
-  fun box() : String {
-    val answer = apply("OK") { String.() : Int ->
-      get(0)
-      length
+
+fun box(): String {
+    val answer = apply("OK") { String.(): Int ->
+        get(0)
+        length()
     }
 
     return if (answer == 2) "OK" else "FAIL"
-  }
+}
 
-  fun apply(arg:String, f :  String.() -> Int) : Int {
-      return arg.f()
-  }
+fun apply(arg: String, f: String.() -> Int): Int {
+    return arg.f()
+}