minor clarifications of the docs (that would have helped me
understand some of the details better).
add docs to address how image equality, rounding, and alpha values
can conspire to confuse

closes PR 13541
fix documentation errors

closes PR 13543
Make mutable lists be sequences.
Move eopl language to Racket.

- sllgen.rkt - use begin-for-syntax+define-syntax instead of
  define-syntax-set.
- eopl.scrbl - rewrite *3-columns so that it no longer errors when 3
  doesn't divide the number of reprovided lang forms.
- Removal of mzscheme and mzlib requires.
- Updated docs.
Newlines at EOFs scan.
Some style things.
racket/draw win32: Pango patch

Closes PR 13513
Revert "racket/draw win32: work around a Pango bug"

This reverts commit 8f9606007fee4caece23214b0b20dc36a3a7c010.

Figured out how to compile and fix Pango after all.
add a contract on the interaction between the module-lexer and the
lexers it defers to

also, remove the checks in color.rkt in the framework (they are not
all covered by the added contract, but they mostly are and when they
aren't, most of those times are using the heavily tested racket-lexer)
Use @racket[] on the unquote-splicing identifier so that we get good hyperlinking.

Closes PR 13548.
Add 'extra' to emphasize the intended meaning.

Closes PR 13550.
fix up tests to reflect the module-lexer now behaves a little
better wrt to its first result

That is, instead of just returning the string "#lang" it now
returns the actual text that was there (it was supposed to be
doing that all along and my recent commit fixed it, but I
missed the test cases)
add has-option? to check if a value has an option contract or not
change the option setup: everyone has an option contract now and
the module lexer either exercises or not, depending on its level of trust
add scrollbars
forgot to add this file in my previous commit
fix bugs in unstable/2d lexer
add a keystroke to widen a rectangle
adjust syntax browser to work better for literal hashes and vectors

closes PR 13537