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/inference/regressions/kt2407.kt

@@ -3,7 +3,7 @@ package n
 import java.util.*
 
 fun test() {
-    val foo = arrayList("").map { it -> it.length }.fold(0, { x, y -> Math.max(x, y) })
+    val foo = arrayList("").map { it -> it.length() }.fold(0, { x, y -> Math.max(x, y) })
     foo : Int
     <!TYPE_MISMATCH!>foo<!> : String
 }

compiler/testData/diagnostics/tests/inference/regressions/kt2484.kt

@@ -8,15 +8,15 @@ fun <T> Array<T>.forEach(operation: (T) -> Unit) : Unit { for (element in this)
 fun bar(operation: (String) -> Unit) = operation("")
 
 fun main(args: Array<String>) {
-    args.forEach { (a : String) : Unit -> a.length } // Type mismatch: (String) -> Unit required, (String) -> Int found
-    args.forEach { (a) : Unit -> a.length } // Type mismatch: (String) -> Unit required, (String) -> Int found
-    args.forEach { (a : String) -> a.length } // Type mismatch: (String) -> Unit required, (String) -> Int found
-    args.forEach { a -> a.length } // Type mismatch: (String) -> Unit required, (String) -> Int found
-    args.forEach { it.length }     // This works!
+    args.forEach { (a : String) : Unit -> a.length() } // Type mismatch: (String) -> Unit required, (String) -> Int found
+    args.forEach { (a) : Unit -> a.length() } // Type mismatch: (String) -> Unit required, (String) -> Int found
+    args.forEach { (a : String) -> a.length() } // Type mismatch: (String) -> Unit required, (String) -> Int found
+    args.forEach { a -> a.length() } // Type mismatch: (String) -> Unit required, (String) -> Int found
+    args.forEach { it.length() }     // This works!
 
-    bar { (a: String) : Unit -> a.length }
-    bar { (a) : Unit -> a.length }
-    bar { (a: String) -> a.length }
-    bar { a -> a.length }
-    bar { it.length }
+    bar { (a: String) : Unit -> a.length() }
+    bar { (a) : Unit -> a.length() }
+    bar { (a: String) -> a.length() }
+    bar { a -> a.length() }
+    bar { it.length() }
 }

compiler/testData/diagnostics/tests/inference/regressions/kt943.kt

@@ -7,8 +7,8 @@ import java.util.*
 
 fun foo(lines: List<String>) {
     val w = max(lines, comparator {o1, o2 ->
-        val l1 : Int = o1.length // Types of o1 and o2 are ERROR
-        val l2 = o2.length
+        val l1 : Int = o1.length() // Types of o1 and o2 are ERROR
+        val l2 = o2.length()
         l1 - l2
     }).sure()
     w : String