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/elvisRHS.kt

@@ -5,6 +5,6 @@
 fun foo(p1: String?, p2: String?) {
     if (p2 != null) {
         val v = p1 ?: <!DEBUG_INFO_SMARTCAST!>p2<!>
-        val size = v.length
+        val size = v.length()
     }
 }