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/builtin-classes.txt

@@ -294,9 +294,8 @@ public final class CharRange : kotlin.Range<kotlin.Char>, kotlin.Progression<kot
 }
 
 public trait CharSequence {
-    public abstract val length: kotlin.Int
-        public abstract fun <get-length>(): kotlin.Int
     public abstract fun get(/*0*/ index: kotlin.Int): kotlin.Char
+    public abstract fun length(): kotlin.Int
 }
 
 public trait Cloneable {
@@ -1429,10 +1428,9 @@ public final class ShortRange : kotlin.Range<kotlin.Short>, kotlin.Progression<k
 
 public final class String : kotlin.Comparable<kotlin.String>, kotlin.CharSequence {
     /*primary*/ public constructor String()
-    public open override /*1*/ val length: kotlin.Int
-        public open override /*1*/ fun <get-length>(): kotlin.Int
     public open override /*1*/ fun compareTo(/*0*/ other: kotlin.String): kotlin.Int
     public open override /*1*/ fun get(/*0*/ index: kotlin.Int): kotlin.Char
+    public open override /*1*/ fun length(): kotlin.Int
     public final fun plus(/*0*/ other: kotlin.Any?): kotlin.String
 }