Games:Remove-mzlib-unitsig-and-cleanup
-Remove all instances of mzlib/unitsig in favor of racket/unit
-Prefer /infer forms.
-move to #lang racket from (module ... racket forms
in the modified games and files.
-remove some lingering scheme imports.
-update documentation

-Paint-by-Numbers:
-move to racket/unit
-update solution and problem gen files to generate using racket/unit
-regen the problem and solution sets using racket/unit
adjust xmethod macro to yield a more helpful error message
extend the lexer <-> framework's color:text api
to let lexers say "call me again before you change the buffer"

also, use this in the 2d lexer
missed some changes related to addition of dont-care values
in the lexers
Two doc typos.

Closes PR 13590.
Closes PR 13591.
adjust 2d reader so the keyword has its own source location
and is syntax-original?
added a keybinding to center a line within its cell in a table
improve the way the font size callbacks work, wrt to interactivity
properly compose readtables
Make vector-set! type correctly on complicated inputs.

Closes PR13532.
Add checks for failing to parse types.
document c:x;r;c
add various dots to tex-table and thus drracket keyboard shortcuts
Adjusted a literal scribble tabular to use 2d
an attempt at 2d documentation
forgot to add tabular.rkt
Fix with-contract when the contracted identifier is defined as syntax.
Look for defines and handle them directly.

Since keyword definitions expand into multiple defines, and inserting
the contract and external id definitions at the wrong point will cause
those to break, we handle define manually.

Hat-tip to Vincent for the idea.

Doing this means that we can't put a single splicing-syntax-parameterize
around the entire body, since it'll expand defines before we reach them,
so instead we add them around the expanded code.
Tests for define/contract and keyword interaction.
fix readtable composition in #2d reader