Move currentSystemDefault() implementation to platform

There's a problem with declaring TZIDVar opaque type as expect and actualizing it with a typealias to another typealias.

Author: ilya-g

core/nativeMain/cinterop_actuals/TimeZoneNative.kt

@@ -4,12 +4,21 @@
  */
 package kotlinx.datetime
 
+import kotlinx.cinterop.*
+import platform.posix.free
+
+internal actual fun getCurrentSystemDefaultTimeZone(): TimeZone = memScoped {
+    val tzid = alloc<kotlinx.datetime.internal.TZIDVar>()
+    val string = kotlinx.datetime.internal.get_system_timezone(tzid.ptr)
+            ?: throw RuntimeException("Failed to get the system timezone.")
+    val kotlinString = string.toKString()
+    free(string)
+    TimeZone(tzid.value, kotlinString)
+}
+
 internal actual fun available_zone_ids(): kotlinx.cinterop.CPointer<kotlinx.cinterop.CPointerVar<kotlinx.cinterop.ByteVar>>? =
         kotlinx.datetime.internal.available_zone_ids()
 
-internal actual fun get_system_timezone(id: kotlinx.cinterop.CValuesRef<kotlinx.datetime.TZIDVar /* = kotlinx.cinterop.ULongVarOf<kotlin.ULong> */>?): kotlinx.cinterop.CPointer<kotlinx.cinterop.ByteVar /* = kotlinx.cinterop.ByteVarOf<kotlin.Byte> */>? =
-        kotlinx.datetime.internal.get_system_timezone(id)
-
 internal actual fun offset_at_datetime(zone: kotlinx.datetime.TZID /* = kotlin.ULong */, epoch_sec: platform.posix.int64_t /* = kotlin.Long */, offset: kotlinx.cinterop.CValuesRef<kotlinx.cinterop.IntVar /* = kotlinx.cinterop.IntVarOf<kotlin.Int> */>?): kotlin.Int =
         kotlinx.datetime.internal.offset_at_datetime(zone, epoch_sec, offset)
 

core/nativeMain/src/TimeZone.kt

@@ -13,12 +13,11 @@ import kotlinx.cinterop.*
 import platform.posix.*
 import kotlin.native.concurrent.*
 
+internal expect fun getCurrentSystemDefaultTimeZone(): TimeZone
 
-internal typealias TZIDVar = kotlinx.cinterop.ULongVarOf<kotlinx.datetime.TZID>
 internal typealias TZID = platform.posix.size_t
 internal expect val TZID_INVALID: TZID
 internal expect fun available_zone_ids(): kotlinx.cinterop.CPointer<kotlinx.cinterop.CPointerVar<kotlinx.cinterop.ByteVar>>?
-internal expect fun get_system_timezone(id: kotlinx.cinterop.CValuesRef<kotlinx.datetime.TZIDVar /* = kotlinx.cinterop.ULongVarOf<kotlin.ULong> */>?): kotlinx.cinterop.CPointer<kotlinx.cinterop.ByteVar /* = kotlinx.cinterop.ByteVarOf<kotlin.Byte> */>?
 internal expect fun offset_at_datetime(zone: kotlinx.datetime.TZID /* = kotlin.ULong */, epoch_sec: platform.posix.int64_t /* = kotlin.Long */, offset: kotlinx.cinterop.CValuesRef<kotlinx.cinterop.IntVar /* = kotlinx.cinterop.IntVarOf<kotlin.Int> */>?): kotlin.Int
 internal expect fun offset_at_instant(zone: kotlinx.datetime.TZID /* = kotlin.ULong */, epoch_sec: platform.posix.int64_t /* = kotlin.Long */): kotlin.Int
 internal expect fun timezone_by_name(zone_name: kotlin.String?): kotlinx.datetime.TZID /* = kotlin.ULong */
@@ -27,14 +26,7 @@ public actual open class TimeZone internal constructor(private val tzid: TZID, a
 
     actual companion object {
 
-        actual fun currentSystemDefault(): TimeZone = memScoped {
-            val tzid = alloc<TZIDVar>()
-            val string = get_system_timezone(tzid.ptr)
-                ?: throw RuntimeException("Failed to get the system timezone.")
-            val kotlinString = string.toKString()
-            free(string)
-            TimeZone(tzid.value, kotlinString)
-        }
+        actual fun currentSystemDefault(): TimeZone = getCurrentSystemDefaultTimeZone()
 
         actual val UTC: TimeZone = ZoneOffset.UTC