Racket: correct font-lock for define/match, again

We weren't considering the obvious: (define/match (foo bar) ...)
For crazy guys that use (define/match [foo bar] ...)

Racket is happy with that, so who are we to disagree?
racket: ,enter submodules

It is now possible to ,enter racket submodules.  This is only the
first part of the story, because evalations should take place in the
submodule, not in its top level module, as it happens now.
racket: ,gcd -> ,pwd

And we display it (the current path, settable via ,cd) as a string.
This was, i'm sure, a secret command nobody is using!
racket: indentation for module*
racket: handling correctly submodules in load handler during ,enter

That is, complying to the submodule loading protocol (cf. racket's own
enter!).
racket: module* and module+ denote submodules too

Recognizing those forms in the elisp module getter.
racket: C-u C-c C-z on a submodule enters it
Nits
racket: new ,geiser-load command in REPL

... and used also internally for C-c C-k, although it doesn't yet work
as well as i wanted when it comes to load modules.  The reason is
probably in geiser/enter, where we don't record modification times per
submodule but per path, which is not correct in the presence of submodules.
racket: struggling with submodules

Submodule (re)loading is not without pecularities.  In particular,
module[*+] submodules are not visited the first time one enters its
parent, but once you load them once, they're revisited every time we
load the parent afterwards--racket's native enter! exhibits the same
behaviour, so i'm guessing we'll have to live with that.

There is however a glitch in that submodules can only be reloaded then
by loading the parent, so we need to confirm that this is expected
behaviour and, if it is, automating the parent's load when the
submodule's is requested.

On the other hand, entering a module[*+] is not working in Geiser yet,
and it does in plain racket, so this one is our fault.  Working on it.
racket: ,cd accepting also non-quoted paths

just because we can