Rewrite the delay-by primitive so it's easier to understand.

Also, add comments that attempt to explain how it's intended to work.
fix compatible-closure to deal properly with language extension

Thanks to David Van Horn for spotting the problem and providing
a nice simple test case

Please include in 6.1
updated HISTORY.txt

Merge to 6.1 release
boldface for performance note
Update TR history.

Please merge to 6.1.
Start filling in RacketCon titles and abstracts.
Commented out some test cases that intentionally failed, so as to not
get spurious failure messages.
heap-remove! sometimes needs to heapify up

closes PR 14651
misc improvements around read-language (uses and docs)

- make a logger for drracket-related #lang problems
- change read-language's docs to indicate that it can return #f
- adjust a bunch of calls to read-language for the case that it returns #f
  with the goal of making that be the same as a get-info function that
  always returns the given default
- mention `read-language` in the guide section entitled
 "Source-Handling Configuration"
More titles and abstracts for RacketCon.
redex: add #:pre to define-judgment-form

also, update contract error messages for the same
redex: fix error messages for #:satisfying

Closes PR 14642
Add `flatten-all-begins` to syntax/flatten-begin
Add doc examples for `flatten-begin`
remove call to 'time'

Please add to 6.1 release branch
fix bug in ->i
fix a test for `raco exe`

The test failed for certain installation configurations due to a
problem with the test.
raco exe: document command-line flags
sync: accept 0 arguments

As suggested by Jonathan Schuster.

Note that the `choice-evt` constructor already accepted 0 arguments.
racket/draw: fix `get-argb-pixels` for unscaled and just-alpha

Closes PR 14653

Merge to v6.1