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)

tests/stdlib/tmemmapstreams.nim

@@ -0,0 +1,53 @@
+discard """
+  file: "tmemmapstreams.nim"
+  output: '''Created size: 10
+Position after writing: 5
+Position after writing one char: 6
+Peeked data: Hello
+Position after peeking: 0
+Readed data: Hello!
+Position after reading line: 7
+Position after setting position: 6
+Readed line: Hello!
+Position after reading line: 7'''
+"""
+import os, streams, memfiles
+const
+  fn = "test.mmapstream"
+var
+  mms: MemMapFileStream
+
+if fileExists(fn): removeFile(fn)
+
+# Create a new memory mapped file, data all zeros
+mms = newMemMapFileStream(fn, mode = fmReadWrite, fileSize = 10)
+mms.close()
+if fileExists(fn): echo "Created size: ", getFileSize(fn)
+
+# write, flush, peek, read
+mms = newMemMapFileStream(fn, mode = fmReadWrite)
+let s = "Hello"
+
+mms.write(s)
+mms.flush
+echo "Position after writing: ", mms.getPosition()
+mms.write('!')
+mms.flush
+echo "Position after writing one char: ", mms.getPosition()
+mms.close()
+
+mms = newMemMapFileStream(fn, mode = fmRead)
+echo "Peeked data: ", mms.peekStr(s.len)
+echo "Position after peeking: ", mms.getPosition()
+echo "Readed data: ", mms.readLine
+echo "Position after reading line: ", mms.getPosition()
+mms.setPosition(mms.getPosition() - 1)
+echo "Position after setting position: ", mms.getPosition()
+
+mms.setPosition(0)
+echo "Readed line: ", mms.readLine
+echo "Position after reading line: ", mms.getPosition()
+
+mms.close()
+
+if fileExists(fn): removeFile(fn)