add io.readChars overload (simpler, less error prone) (#16044)

* add simpler to use readChars overload

* use new readChars overload

* Update lib/wrappers/openssl.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: flywind <xzsflywind@gmail.com>

tests/stdlib/tio.nim

@@ -0,0 +1,37 @@
+# xxx move to here other tests that belong here; io is a proper module
+
+import std/os
+from stdtest/specialpaths import buildDir
+
+block: # readChars
+  let file = buildDir / "D20201118T205105.txt"
+  let s = "he\0l\0lo"
+  writeFile(file, s)
+  defer: removeFile(file)
+  let f = open(file)
+  defer: close(f)
+  let n = f.getFileInfo.blockSize
+  var buf = newString(n)
+  template fn =
+    let n2 = f.readChars(buf)
+    doAssert n2 == s.len
+    doAssert buf[0..<n2] == s
+  fn()
+  setFilePos(f, 0)
+  fn()
+
+  block:
+    setFilePos(f, 0)
+    var s2: string
+    let nSmall = 2
+    for ai in buf.mitems: ai = '\0'
+    var n2s: seq[int]
+    while true:
+      let n2 = f.readChars(toOpenArray(buf, 0, nSmall-1))
+      # xxx: maybe we could support: toOpenArray(buf, 0..nSmall)
+      n2s.add n2
+      s2.add buf[0..<n2]
+      if n2 == 0:
+        break
+    doAssert n2s == @[2,2,2,1,0]
+    doAssert s2 == s