Add #:reduce argument to traces.

Add a new #:reduce argument to traces (and traces/ps) that defines the
function to use when applying the reduction relation.  This way a
function could selectively display some subset of possible reduction
traces instead of all.
Add an example to the Redex typesetting docs that discusses how
macros screw up syntax locations and what to do about it

Thanks to Lindsey Kuper for the inspiration to do this
Document procedure-closure-contents-eq?.
Memoize wrapped case-> range contracts.

Fixes failing contract tests.
raco pkg: support single-collection packages

A package directory is in single-collection mode when it has an
"info.rkt" file containing a `single-collection' definition.
The string value of `single-collection' provides the collection's
name.
Add 'no-sidebar style property as a style for the main part of a document

The new style property causes the HTML output to not include an ``on this
page'' margin box.
doc repair

Fix "style" where it should be "style property".
ffi/unsafe: doc repair

from Клочков Роман
Move the contract profiler to its own collection.

Should become its own package once we split up the collects.

Requested by Eli, to reduce the dependencies of the profiler collection.
Add match*? to unstable/match
Fix union merging

Trying to merge (and thus resolve) applications of struct
types would cause infinite looping on type instantiation
if the struct type used both a union and recursion.

Closes PR 13821
Improve TR test case
Fix type of gensym.
Fix dist-specs.
Remove leftover define/match docs
Superficial pattern for wildcard that doesn't bind.
add some missing pref items docs
add a column-width guide; defaults to 102 characters, but disabled
added history not about column width guide
fix `hash-copy' on weak hash tables

Closes PR 13838