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/delegatedProperty/propertyMetadataShouldBeCached.kt

@@ -34,7 +34,7 @@ object StringHandler {
 fun box(): String {
     val a = A()
     a.foo = 42
-    a.foo = a.foo + baz.length
+    a.foo = a.foo + baz.length()
     a.foo = 239
     A.bar = baz + a.foo
     baz + A.bar