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/boxInline/complex/closureChain.1.kt

@@ -1,11 +1,10 @@
-
 fun test1(): Int {
     val inlineX = Inline()
-    return inlineX.foo({(z: Int) -> "" + z}, 25, {String.() -> this.length})
+    return inlineX.foo({(z: Int) -> "" + z}, 25, {String.() -> this.length()})
 }
 
 fun box(): String {
     if (test1() != 2) return "test1: ${test1()}"
 
     return "OK"
-}
+}