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/strings/kt3571.kt

@@ -0,0 +1,6 @@
+class Thing(delegate: CharSequence) : CharSequence by delegate
+  
+fun box(): String {
+    val l = Thing("hello there").length()
+    return if (l == 11) "OK" else "Fail $l"
+}

compiler/testData/codegen/box/strings/multilineStringsWithTemplates.kt

@@ -14,7 +14,7 @@ fun box() : String {
 
     val test5 =
 """
-  ${s.length}
+  ${s.length()}
 """
     if (test5 != "\n  3\n") return "Fail 5: $test5"