Add MemMapFileStream. Fixes in memFiles. (#7944)

* Add MemMapFileStream

* Added tests

* Fixed bug in memfiles (zero index for string)

* Added flush to changelog

* Attempt to fix Win's nuances

* Fix attempt to fix

* Continue...

* And again...

* Reworked tests (all for win on Win)

* Fixes in flush (Win)

* Replace fn vars to consts

* Added the attempts parameter to the flush

* Replace while to for

* Move to memfiles

* Use Natural instead of uint

* Better error messages for append mode. Handle specific cases.

tests/stdlib/tmemfiles2.nim

@@ -4,9 +4,10 @@ discard """
 Half read size: 10 Data: Hello'''
 """
 import memfiles, os
+const
+  fn = "test.mmap"
 var
   mm, mm_full, mm_half: MemFile
-  fn = "test.mmap"
   p: pointer
 
 if fileExists(fn): removeFile(fn)