My first PLT patch

I've submitted my first patch to the internals of PLT Scheme. It's a tiny thing: the ability to unquote inside a quasiquoted hash-table literal, e.g.:

> `#hash((x . ,(+ 1 2)))
#hash((x . 3))

And Matthew had to fix up some issues with it.

But anyway, making contributions to PLT is easier these days due to a couple of factors. First, the creation of plt-dev, a public mailing list for discussing the development of PLT Scheme, means that there's better support for people who want to contribute. Second, I'm told that more of the internals of PLT Scheme are self-hosted than apparently they used to be, i.e. a lot of PLT Scheme is itself implemented in PLT Scheme. This makes it a lot less scary to dive into the code.