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/QualifiedExpressions.txt

@@ -1,6 +1,6 @@
 package
 
 package qualified_expressions {
-    internal fun test(/*0*/ s: kotlin.String?): kotlin.Unit
+    internal fun test(/*0*/ s: kotlin.IntRange?): kotlin.Unit
     internal fun kotlin.String.startsWith(/*0*/ s: kotlin.String): kotlin.Boolean
 }