1. &io.Data may not be a good abstraction 2. Writer interface

[bobzhang]

1. From what I can tell, it only supports 3 kinds of data: Bytes, String and Json where String and Json are used most. It was introduced to allow some overloading so that

@http.post("url", data)
@http.post("url", { "xx" : "json"})

but this does not work in practice so that we have to write @http.post("url", ({"xx" : "json"} : Json)) which is not better than
@http.post_json("url", {"x" : "data"}) when it could be more efficient or more redable.

it looks to me that either

@http.post("url", String("..."))
@http.post("url", Bytes("..."))
@http.post("url", Json("..."))

or

@http.post("url","...")
@http.post_bytes("url","...")
@http.post_json("url", "...")

they are both more readable and can be more efficient than the current interface
2.

pub(open) trait Writer {
async write_once(Self, Bytes, offset~ : Int, len~ : Int) -> Int // --> async write_once(Self, BytesView) -> Int
async write(Self, &Data) -> Unit = _
async write_reader(Self, &Reader) -> Unit = _
#deprecated("use write instead", skip_current_package=true)
async write_string(Self, StringView, encoding~ : Encoding) -> Unit = _
}