Support for not showing status code from shell commands

[ALSchwalm]

Fixes #161

This mostly just pulls in the run-context logic from janet. It sounds like we were probably going to need that anyway for #194 (though I might misunderstand, I've not been closely tracking that).

Adds a *show-result* variable that defaults to true. When set to nil, the output from evaluations is not shown when implicit parens have been added.

[andrewchambers]

Yeah, the idea with #194 is that this let's us add a hook before each top level form is evaluated and do some introspection on the form body.

Not suggesting a change, but I wonder if the show-result login could be part of a default hook.

[ALSchwalm]

Maybe? I think we would probably run in to some issues though. Namely, we would need to know in the post-exec hook whether we had added parens in the pre-exec hook and getting that info there seems like it would probably be messy. But I could definitely be wrong.

[andrewchambers]

I wonder if show-result should be something like show-exit-code or show-exit-status

[ALSchwalm]

Yep, that sounds better. I'll change it. The other question is if we want to store the exit code in a variable (I would assume this would be done regardless of the value of 'show-exit-code'), but I could make a separate issue/pr for that.

[andrewchambers]

The old repl stored them in '_' , you can see janet does this after pp in the old signal handler.

[ALSchwalm]

Ah, true. I'll add that and rebase

[andrewchambers]

Oh it seems this new repl also doesn't print a stack trace like the old one when we error.

[ALSchwalm]

Should be fixed now. Sorry about that.

[andrewchambers]

Thank you very much :)