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/compileKotlinAgainstKotlin/PropertyReference.A.kt

@@ -3,4 +3,4 @@ package a
 public var topLevel: Int = 42
 
 public val String.extension: Long
-    get() = length.toLong()
+    get() = length().toLong()