Generate sum() methods for arrays, iterables and sequences of Byte and Short. These overloads got explicit platformName.

Heuristics to decide when to set platformName.
#KT-3714

libraries/stdlib/src/generated/_Generators.kt

@@ -5,6 +5,7 @@ package kotlin
 // See: https://github.com/JetBrains/kotlin/tree/master/libraries/stdlib
 //
 
+import kotlin.platform.*
 import java.util.*
 
 import java.util.Collections // TODO: it's temporary while we have java.util.Collections in js