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
Working with links in collection testing
futures: bug fix

The problem was in the handing of "lightweight continuations" used to
represent suspended computations, and in particular the handling
of continuation marks (such as the ones added by DrRacket's debugging
mode).

Unfortunately, I wasn't able to construct a simpler test case that
triggers the problem. I think the number of installed continuation marks
has to be just right, and there have to be some replacing marks, and
a lightweight continuaiton has to be captured at the right time
as a result of applying a previously captured continuation.

Closes PR 13427

Merge to v5.3.2
Gracefully handle the case where the token stream just contains the END token and an error occurs.
fix missing argument
  closes PR 13429

Merge to 5.3.2
Fix XREPL logging printer to handle new logger name field.

Merge to 5.3.2.
Fix parsing of `maybe-optimize` result.

Merge to 5.3.2.
Abstract out OC sandboxing.
Log output generated when OC expands/runs programs.

To be consistent with Check Syntax.
Abstract out right file filtering.
Log functions too big to be inlined.

Patch from Matthew.
First stab at hooking the profiler up to OC.

Profiling is done from inside the eval, to minimize noise.

Use samplers directly, to get access to the profiler's data structures.
Put sandbox management in its own file.

To avoid circular dependencies.
Refactoring.
Only keep profile reports from the relevant file.
Propagate profile information to inlining analysis.