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/DoubleDefine.kt

@@ -9,7 +9,7 @@ fun takeFirst(expr: StringBuilder): Char {
 }
 
 fun evaluateArg(expr: CharSequence, numbers: ArrayList<Int>): Int {
-  if (expr.length == 0) throw Exception("Syntax error: Character expected");
+  if (expr.length() == 0) throw Exception("Syntax error: Character expected");
   val c = takeFirst(<error>expr</error>)
   if (c >= '0' && c <= '9') {
     val n = c - '0'