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/controlStructures/kt3273.kt

@@ -12,7 +12,7 @@ public fun testCoalesce() {
         else -> "Hello world"
     }
 
-    printlnMock(value.length)
+    printlnMock(value.length())
 }
 
 fun box(): String {