implements std/streams

[ringabout]

ref #1534

std/streams is required by std/osproc

[ringabout]

type
  StreamObj* = object of RootObj
    closeImpl*: proc (s: Stream)
      {.nimcall, raises: [IOError, OSError], tags: [WriteIOEffect].}

proc ssClose(s: Stream) =
  var s = StringStream(s)
  s.data = ""

result.closeImpl = ssClose

now gives an error

Error: type mismatch: got: proc (s: ref StreamObj) but wanted: proc (s: ref StreamObj) {.nimcall, raises: [IOError, OSError], tags: [WriteIOEffect].}

[ringabout]

There is still a bug about StringStream in #1561, but should be enough for std/osproc. Good news is that we can do without std/streamwrapper since these functions are only used by peekableOutputStream etc. The documentation will be added after the bug is fixed