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/resolve/nestedCalls/kt5971NestedSafeCall.kt

@@ -5,5 +5,5 @@
 fun foo(i: Int) {}
 
 fun test(s: String?) {
-    foo(<!TYPE_MISMATCH!>s?.length<!>)
-}
+    foo(<!TYPE_MISMATCH!>s?.length()<!>)
+}