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/diagnostics/functions/tailRecursion/realStringEscape.kt

@@ -6,11 +6,10 @@ fun escapeChar(c : Char) : String? = when (c) {
 }
 
 tailRecursive fun String.escape(i : Int = 0, result : StringBuilder = StringBuilder()) : String =
-        if (i == length) result.toString()
+        if (i == length()) result.toString()
         else escape(i + 1, result.append(escapeChar(get(i))))
 
 fun box() : String {
     "test me not \\".escape()
     return "OK"
 }
-