add memfiles test of create/read/write data

tests/stdlib/tmemfiles2.nim

@@ -0,0 +1,38 @@
+discard """
+  test creating/reading/writing/changing memfiles
+  file: "tmemfiles2.nim"
+  output: '''Full read size: 20
+Half read size: 10 Data: Hello'''
+"""
+import memfiles, os
+var
+  mm, mm_full, mm_half: MemFile
+  fn = "test.mmap"
+  p: pointer
+
+if fileExists(fn): removeFile(fn)
+
+# Create a new file, data all zeros
+mm = memfiles.open(fn, mode = fmReadWrite, newFileSize = 20)
+mm.close()
+
+# read, change
+mm_full = memfiles.open(fn, mode = fmWrite, mappedSize = -1)
+echo "Full read size: ",mm_full.size
+p = mm_full.mapMem(fmReadWrite, 20, 0)
+var p2 = cast[cstring](p)
+p2[0] = 'H'
+p2[1] = 'e'
+p2[2] = 'l'
+p2[3] = 'l'
+p2[4] = 'o'
+p2[5] = '\0'
+mm_full.unmapMem(p, 20)
+mm_full.close()
+
+# read half, and verify data change
+mm_half = memfiles.open(fn, mode = fmRead, mappedSize = 10)
+echo "Half read size: ",mm_half.size, " Data: ", cast[cstring](mm_half.mem)
+mm_half.close()
+
+if fileExists(fn): removeFile(fn)