Use #lang r7rs for the Racket prelude

[lexi-lambda]

This changes the Racket prelude to use #lang r7rs, which appears to be able to run all the benchmarks without errors (except for one—mentioned below). However, there are some important caveats:

• Using #lang r7rs requires installing the r7rs package, which is not distributed with Racket. If you want to try going down this route, you should probably add a note in the README as you see fit.
• Currently, #lang r7rs is implemented as an attempt at strict compliance, which means it might suffer some performance hits in a few areas. I am considering implementing a #lang r7rs/interop language that is a bit less strict but would improve Racket interop and would likely be speedier.
• The nqueens.scm test does not run, but I am pretty sure there is a syntax error that is not valid in any Scheme standard I know of: an if has 4 subforms.

Maybe just give it a try and see if it works to your liking? I figured I’d at least put this pull request together to show you what it would look like (since it’s so straightforward, anyway).

[ecraven]

Thanks, looks good, merging :) The 4-handed if is my fault, I was fixing one-handed ifs to when/unless, and probably broke this one.