Add css-(un)escape, allow using escaped selectors

[aartaka]

This adds:

• An ability to escape CSS strings (css-escape, API akin to CSS.escape()) and parse escaped ones back to literal values (css-unescape, a naïve reverse of css-escape).
• Simplified identifier parsing, as per the CSS spec, instead of unrelated and more restrictive HTML spec.
• The extension of the clss-name syntax rule to allow backslash-escaped characters (should it be #x1F to #xFF, actually? I've included all of ASCII just in case...)
• The intuitive parsing of those in the matchers.

All in all, it fixes #13 (if you use css-escape on the class name) and allows for a safer and more reliable element selection.

@Shinmera, I've tried to optimize the hell out of css-escape and css-unescape, but I may have missed some important nit. For instance, how do you specify the element type of a string -- it should be character, right? SBCL complains about it being unspecified :/

[aartaka]

Should be all addressed now:

• I've used do&cond in favor of loop.
• css-unescape is now called when compiling selectors, not matching them.
• escapable is a toplevel function now.

[aartaka]

I've introduced some index-out-of-bounds mistakes while refactoring, they should be fixed now :P

[Shinmera]

Looks good (save for not upcasing t, but I'll let that slide for now.)

[aartaka]

Ahhhh, right, I'm always forgetting about this stylistic convention of yours 😅