Safe(r) mkdir and touch

[chrisosaurus]

Attempting to address #33

I have defined a set of characters allowed within mkdir and touch, both will refuse any string which isn't made up entirely of those characters.

I have also changed boot.lisp to load builtins before core, so that core can use builtins in the implementation - I think this makes sense.

[eriksvedang]

Cool, I'll merge this when I'm back at my computer tomorrow or Monday.

[ghost]

What about spaces and special characters (as pointed out by @lieuwex)?

[chrisosaurus]

@shyguy as mentioned on #33 I was planning to add those later, happy to drop in favour of #43 if it is still safe.

[chrisosaurus]

I'm breaking out the other parts of this commit in preparation for this being closed.
#44 - changing boot ordering (builtins before core)
#45 - string contains

[chrisosaurus]

rebased and squashed down.

depends on #44 and #45 landing first.

[chrisosaurus]

Closing in favour of #43