Support ! commands in REPL.

[porcuquine]

When #180 introduced self-evaluating keyword symbols, the :load <filename> 'command' behavior in the REPL was broken.

This PR replaces that functionality by supporting the !(:load <filename>) syntax. It also makes all the other !(:command …) forms available at the REPL (not just when loading or running files).

This is a divergence from Lisp Lurk's REPL, which will probably not be updated to match the new (preferred) behavior.

[weissjeffm]

Aside from the question below, looks good to me.

Is this a common-lisp 'ism? !(:foo bar) (where the meta function is invoked using a keyword like :foo instead of the usual symbol foo?)

If it is then I think it's ok to reuse as users would likely recognize what it means. If it isn't (and it's new syntax we're creating), we might want to consider using a symbol there instead. My rationale would be that we already know we're invoking the meta-language from the ! syntax, and we don't need further indication that the function isn't a lisp function.

You could argue that we want a completely separate namespace so that we can later mix calls to :foo and foo which are different things. However I don't know if that is the real intent (to be able to mix meta function calls with function calls).

[weissjeffm]

See my comment but it will not block approval.

[porcuquine]

I generally agree that these should eventually be normal symbols (perhaps in a namespace like .lurk.meta.). We might even want to lose the !(…), but there are issues to consider.

Regardless, this PR isn't intended to modify the syntax already used when loading and running Lurk files. It just extends the interactive REPL to use those same commands.

Once better package-handling is done, we can (strongly) consider this.