Let handles be seen outside of memfiles module so that "updating"

operations (like eg., resizing a file and re-mapping) do not need
to worry about race conditions of re-opened paths, renamed parent
directories and that sort of thing.  Operating directly on already
open handles is both safer and more efficient than relying upon the
stability of filesystem paths.

lib/pure/memfiles.nim

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