Command I/O via sockets

Send commands and receive responses via sockets.

Fixes #132.
Fixes #203.
Fixes #228.

Uses some code from the old, stale `old-command-socket` branch.

Also:

- Some commands -- like profiling and test coverage -- need to
  racket-run, then issue a command to get results about the run. Provide
  a way for them to see if the REPL is waiting at the Racket prompt.

- Make the previous point for racket-check-syntax-mode by simplifying it
  to supply a path.

- Separate find and kill of requires. Wait to kill until we're ready to
  insert the new requires. This simplifies error recovery. Also it means
  something like `accept-process-output' won't make a new undo history
  point, so both our kill and insert will be in a single undo step.

Reference.md

@@ -666,18 +666,6 @@ Macro expand again the previous expansion done by one of:
 - [`racket-expand-last-sexp`](#racket-expand-last-sexp)
 - [`racket-expand-again`](#racket-expand-again)
 
-### racket-gui-macro-stepper
-<kbd>M-x racket-gui-macro-stepper</kbd>
-
-Run the DrRacket GUI macro stepper.
-
-Runs on the active region, if any, else the entire buffer.
-
-EXPERIMENTAL: May be changed or removed.
-
-BUGGY: The first-ever invocation might not display a GUI window.
-If so, try again.
-
 # Variables
 
 > Note: You may also set these via Customize.
@@ -687,6 +675,12 @@ If so, try again.
 ### racket-racket-program
 Pathname of the racket executable.
 
+### racket-command-port
+Port number for Racket REPL command server.
+
+### racket-command-timeout
+Timeout for Racket REPL command server.
+
 ### racket-raco-program
 Pathname of the raco executable.
 
@@ -727,7 +721,15 @@ List of command-line arguments to supply to your Racket program.
 Accessible in your Racket program in the usual way -- the
 parameter `current-command-line-arguments` and friends.
 
-The value must be an unquoted list of strings such as:
+This is an Emacs buffer-local variable -- convenient to set as a
+file local variable. For example at the end of your .rkt file:
+
+    ;; Local Variables:
+    ;; racket-user-command-line-arguments: ("-f" "bar")
+    ;; End:
+
+Set this way the value must be an unquoted list of strings such
+as:
 
     ("-f" "bar")
 
@@ -736,13 +738,6 @@ but NOT:
     '("-f" "bar")
     (list "-f" "bar")
 
-This is an Emacs buffer-local variable -- convenient to set as a
-file local variable. For example at the end of your .rkt file:
-
-    ;; Local Variables:
-    ;; racket-user-command-line-arguments: ("-f" "bar")
-    ;; End:
-
 
 ## REPL
 

channel.rkt

@@ -4,7 +4,7 @@
          racket/contract
          "mod.rkt")
 
-(provide the-channel
+(provide main-channel
          (struct-out msg)
          (struct-out load-gui)
          (struct-out rerun)
@@ -42,8 +42,8 @@
 (define (profile/coverage-level? v)
   (memq? v profile/coverage-levels))
 
-;; Messages via a channel from the repl thread to the main thread.
-(define the-channel (make-channel))
+;; Messages to the main thread via a channel
+(define main-channel (make-channel))
 (define-struct/contract msg ())
 (define-struct/contract [load-gui msg] ())
 (define-struct/contract [rerun msg]
@@ -63,5 +63,5 @@
 ;; the channel, and blocks itself; main thread will kill the REPL
 ;; thread. Effectively "exit the thread with a return value".
 (define (put/stop v) ;; msg? -> void?
-  (channel-put the-channel v)
+  (channel-put main-channel v)
   (void (sync never-evt)))