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/kt310.txt

@@ -1,3 +1,3 @@
 package 
 
-internal fun f(/*0*/ c: kotlin.String): kotlin.Int
+internal fun f(/*0*/ c: kotlin.LongRange): kotlin.Int