prepend_default_steps: Replace captures with MFArgs

[wojtekmach]

Currently our request struct looks like this:

iex> Req.build(:get, "https://api.github.com") |> Req.prepend_default_steps()
%Req.Request{
  request_steps: [#Function<0.30967734/1 in Req."-fun.normalize_headers/1-">,
   #Function<1.30967734/1 in Req."-fun.default_headers/1-">,
   #Function<2.30967734/1 in Req."-fun.encode/1-">],
  response_steps: [#Function<9.30967734/2 in Req."-fun.follow_redirects/2-">,
   #Function<10.30967734/2 in Req."-fun.decompress/2-">,
   #Function<11.30967734/2 in Req."-fun.decode/2-">],
  ...
}

let's change add_default_steps/2 to use MFArgs so that request, response, and error steps are easier to understand:

%Req.Request{
  request_steps: [
    {Req, :normalize_headers, []},
    {Req, :default_headers, []},
    ...
  ],
  ...
}

we'll of course still continue to support passing anonymous functions.