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/controlFlowAnalysis/definiteReturnInWhen.kt

@@ -1,6 +1,6 @@
 fun illegalWhenBlock(a: Any): Int {
     when(a) {
         is Int -> return <!DEBUG_INFO_SMARTCAST!>a<!>
-        is String -> return <!DEBUG_INFO_SMARTCAST!>a<!>.length
+        is String -> return <!DEBUG_INFO_SMARTCAST!>a<!>.length()
     }
-<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
+<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>