Hide auto-started REPL

I'm not convinced that this is the right thing, and the effect is a
bit ugly (we use save-window-excursion), but maybe this is the correct
thing to do for users that want auto-start.
Evaluate last expression to buffer with a prefix

Use a prefix before pressing C-x C-e to print the result of evaluating
the expression before mark to the current buffer.
Simplifications to previous patch
A bit of documentation for the previous patches
Racket: accepting spaces within comint's prompt

Since spaces are allowed inside filenames after all.
Good NEWS
Another take at the fix (files with >, bleh)
A better solution to the funky filename problem

So, the problem was that our regexp for a Racket prompt didn't take
into account that filenames could contain white spaces: "@[^ ]*> ".  A
simple solution was accepting them: "@[^>]+> " won't work because '>'
is also a valid character in filenames, so we went for "@.*> ".

The drawback is that finding the beginning of the prompt (e.g. in C-a)
fails when you're writing things like:

   racket@foo bar.rkt> (> 2 3)

because here comint believes that the prompt is "racket@foo bar.rkt> (> "

And that could have side-effects elsewhere.  So what i've done is
simply changing the way white-space is (not) printed in the prompt,
substituting it by underscores.  That way, whe can go back to the
initial regexp, comint doesn't get confused, and users can easily
infer that "@foo_bar.rkt>" is actually referring to their
"foo bar.rkt" file.