Simplify http-request mods

[orthecreedence]

I think there may be a way to perform very minimal mods to drakma:http-request and achieve the same result using the :continuation method. Would be worth investigating, because then future versions of drakma could be ported simply (or even automatically).

[orthecreedence]

Incorrect, because I would have to give up :continuation support, and I'd rather be 100% API compatible with the real drakma.

Also, the conversion process is now 100% automated (save for copying and pasting http-request into hijack.lisp, and there's no real need for more simplification. The changes made are fairly essential (and already somewhat minimal).