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/labels/kt4247.kt

@@ -2,10 +2,10 @@
 
 fun foo(bar1: (String.() -> Int) -> Int) {
     bar1 {
-        this.length
+        this.length()
     }
 
     bar1 {
-        this@bar1.length
+        this@bar1.length()
     }
 }