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/diagnostics/tests/smartCasts/kt5455.kt

@@ -1,7 +1,7 @@
 //KT-5455 Need warning about redundant type cast
 fun foo(o: Any): Int {
     if (o is String) {
-        return (o <!USELESS_CAST_STATIC_ASSERT_IS_FINE!>as<!> String).length
+        return (o <!USELESS_CAST_STATIC_ASSERT_IS_FINE!>as<!> String).length()
     }
     return -1
 }