Address dom96/Araq opinions in https://github.com/nim-lang/Nim/pull/9922

Updating accessors are also provided since the idea of this change is to
allow "updating" operations external to the module which are by their very
nature closely tied to module internals (as well as to OS interface details).

lib/pure/memfiles.nim

@@ -36,11 +36,26 @@ type
     size*: int       ## size of the memory mapped file
 
     when defined(windows):
-      fHandle*: Handle
+      fHandle: Handle
-      mapHandle*: Handle
+      mapHandle: Handle
-      wasOpened*: bool   ## only close if wasOpened
+      wasOpened: bool   ## only close if wasOpened
     else:
-      handle*: cint
+      handle: cint
+
+when defined(windows):
+  proc fHandle*(m: MemFile): Handle = m.fHandle
+  proc mapHandle*(m: MemFile): Handle = m.mapHandle
+  proc wasOpened*(m: MemFile): bool = m.wasOpened
+  proc `fHandle=`*(m: var MemFile, fHandle: Handle) =
+    m.fHandle = fHandle
+  proc `mapHandle=`*(m: var MemFile, mapHandle: Handle) =
+    m.mapHandle = mapHandle
+  proc `wasOpened=`*(m: var MemFile, wasOpened: bool) =
+    m.wasOpened = wasOpened
+else:
+  proc handle*(m: MemFile): cint = m.handle
+  proc `handle=`*(m: var MemFile, handle: cint) =
+    m.handle = handle
 
 proc mapMem*(m: var MemFile, mode: FileMode = fmRead,
              mappedSize = -1, offset = 0): pointer =