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

idea/testData/quickfix/expressions/afterUnnecessaryNonNullAssertion3.kt

@@ -1,4 +1,4 @@
 // "Remove unnecessary non-null assertion (!!)" "true"
 fun test(value : String) : Int {
-    return value<caret>.length
+    return value<caret>.length()
 }

idea/testData/quickfix/expressions/beforeUnnecessaryNonNullAssertion3.kt

@@ -1,4 +1,4 @@
 // "Remove unnecessary non-null assertion (!!)" "true"
 fun test(value : String) : Int {
-    return value<caret>!!.length
+    return value<caret>!!.length()
 }