Use Racket lexer for code blocks.

Had been specifying Scheme lexer for code blocks, while waiting for
new Racket lexer to wend its way from Pygments to Pygments.rb to
Linguist to GitHub.

That day is almost here: Linguist will soon update and deploy to
GitHub.  And Racket 5.3.2 is about to release.  As a result, I think
this is the correct time to switch to the Racket lexer: It should be
live on GitHub by the time people are using Racket 5.3.2.

/cc @rmculpepper -- I think this commit should go into the 5.3.2
release.
scribble/decode: fix contract on `decode-compound-paragraph'

Add a test suite that at least tries each export of `scribble/decode'.

Merge to v5.3.2
scribble doc corrections

Merge to v5.3.2
readline/readline: add `readline-newline' and `readline-redisplay'

mzrl.rkt:
  Add and provide readline-newline and readline-readisplay.

readline.scrbl:
  Documentation and example code.
It fixes a crash on OpenBSD

It fixes a crash in the installation on OpenBSD. Racket reaches the
limits of the shell when it is compiled with a non-root user account.

Tested on OpenBSD-current amd64.

Merge to v5.3.2