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/frontend/src/org/jetbrains/jet/lang/evaluate/OperationsMapGenerated.kt

@@ -100,6 +100,7 @@ private val unaryOperations: HashMap<UnaryOperationKey<*>, Pair<Function1<Any?,
     unaryOperation(SHORT, "toLong", { a -> a.toLong() }, emptyUnaryFun),
     unaryOperation(SHORT, "toShort", { a -> a.toShort() }, emptyUnaryFun),
     unaryOperation(SHORT, "toString", { a -> a.toString() }, emptyUnaryFun),
+    unaryOperation(STRING, "length", { a -> a.length() }, emptyUnaryFun),
     unaryOperation(STRING, "toString", { a -> a.toString() }, emptyUnaryFun)
 )