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/codegen/box/diagnostics/functions/tailRecursion/realStringRepeat.kt

@@ -4,5 +4,5 @@ tailRecursive fun String.repeat(num : Int, acc : StringBuilder = StringBuilder()
 
 fun box() : String {
     val s = "a".repeat(10000)
-    return if (s.length == 10000) "OK" else "FAIL: ${s.length}"
-}
+    return if (s.length() == 10000) "OK" else "FAIL: ${s.length()}"
+}