Streaming file support
I am wondering if you will drop support for httpbeast for netkit?

I have a pull request that will stream large files, its a simple change but it won't fix the memory leak with httpbeast. Additionally the content-length is sent twice because it appears httpbeast is loading the body data into its own buffer before sending it, so I corrected this with `when defined`, but the content length shows as 0.

On macOS it will serve a 600mb and the executable will use 4.9MB memory afterwards.

Please let me know if you would like me to create a pull request.
`context.nim`
```nim

  else:
    when defined(windows) or defined(usestd):
      ctx.response.setHeader("Content-Length", $contentLength)
    await ctx.respond(Http200, "", headers)
    var
      file = openAsync(filePath, fmRead)

    while true:
      let value = await file.read(4096)
      if value.len > 0:
        await ctx.send(value)
      else:
        break

    file.close()
    ctx.handled = true
```