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/dataFlowInfoTraversal/kt4332WhenBranches.kt

@@ -3,7 +3,7 @@
 fun testWhen(t: String?, x: String?): Int {
     return when {
         t == null -> 0
-        x == null -> <!DEBUG_INFO_SMARTCAST!>t<!>.length // Wrong error report here. t can be inferred as not-null. (And it actually does if you replace when with if/else if)
-        else -> (<!DEBUG_INFO_SMARTCAST!>t<!> + x).length
+        x == null -> <!DEBUG_INFO_SMARTCAST!>t<!>.length() // Wrong error report here. t can be inferred as not-null. (And it actually does if you replace when with if/else if)
+        else -> (<!DEBUG_INFO_SMARTCAST!>t<!> + x).length()
     }
-}
+}