system refactorings (#10559)

* move IO subsystem into its own module; refs #10385
* make standalone test compile again
* make C++ examples compile again
* make more tests green
* make sysAssert and gcAssert work again

lib/system/gc_ms.nim

@@ -88,7 +88,8 @@ when not defined(useNimRtl):
 template gcAssert(cond: bool, msg: string) =
   when defined(useGcAssert):
     if not cond:
-      echo "[GCASSERT] ", msg
+      cstderr.rawWrite "[GCASSERT] "
+      cstderr.rawWrite msg
       quit 1
 
 proc cellToUsr(cell: PCell): pointer {.inline.} =