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/combineWithNoSelectorInfo.kt

@@ -3,7 +3,7 @@ package foo
 fun dispatch(request: Request) {
     val <!UNUSED_VARIABLE!>url<!> = request.getRequestURI() as String
 
-    if (request.getMethod()?.length != 0) {
+    if (request.getMethod()?.length() != 0) {
     }
 }