Remove qstring dependency. (#221)

Instead, rely on Url's built-in query parameter handling. A Request now
accumulates a list of query param pairs, and joins them with a parsed
URL at the time do_call is called.

In the process, remove some getters that rely on parsing the URL.
Adapting these getters was going to be awkward, and they mostly
duplicate things people can readily get by parsing the URL.

Cargo.toml

@@ -28,7 +28,6 @@ base64 = "0.13"
 chunked_transfer = "1.2.0"
 cookie = { version = "0.14", features = ["percent-encode"], optional = true}
 once_cell = "1"
-qstring = "0.7"
 url = "2"
 socks = { version = "0.3.2", optional = true }
 rustls = { version = "0.18", optional = true, features = [] }