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/regressions/DoubleDefine.kt

@@ -1,4 +1,3 @@
-
 import java.util.*
 
 import java.io.*
@@ -10,7 +9,7 @@ fun takeFirst(expr: StringBuilder): Char {
 }
 
 fun evaluateArg(expr: CharSequence, numbers: ArrayList<Int>): Int {
-  if (expr.length == 0) throw Exception("Syntax error: Character expected");
+  if (expr.length() == 0) throw Exception("Syntax error: Character expected");
   val c = takeFirst(<!TYPE_MISMATCH!>expr<!>)
   if (c >= '0' && c <= '9') {
     val n = c - '0'

compiler/testData/diagnostics/tests/regressions/Jet121.kt

@@ -3,7 +3,7 @@ package jet121
 fun box() : String {
 val answer = apply("OK") { String.() : Int ->
   get(0)
-  length
+  length()
 }
 
 return if (answer == 2) "OK" else "FAIL"

compiler/testData/diagnostics/tests/regressions/kt549.kt

@@ -11,7 +11,7 @@ package demo
   }
 
 fun main(args : Array<String>) {
-  for (a in filter(args, {it.length > 1})) {
+  for (a in filter(args, {it.length() > 1})) {
     System.out.println("Hello, ${a}!")
   }
 }

compiler/testData/diagnostics/tests/regressions/kt580.kt

@@ -11,9 +11,9 @@ fun main() {
     val diffs = ArrayList<Int>()
     for (i in vals.indices) {
         for (j in i..vals.lastIndex())  // Type inference failed
-             diffs.add(vals[i].length - vals[j].length)
+             diffs.add(vals[i].length() - vals[j].length())
         for (j in i..vals.lastIndex)  // Type inference failed
-             diffs.add(vals[i].length - vals[j].length)
+             diffs.add(vals[i].length() - vals[j].length())
     }
 }