Address feedback from plenary

[bakkot]

Commits should be reviewed individually. Summary:

• use \$ etc where possibly; this is not possible for all punctuators, but it is for some
• also escape line terminators; this was just an oversight on my part
• use \n etc where possible
• if given a lone surrogate, escape it, so that if there is someday a non-BMP whitespace character, and we get x-mode RegExps, and someone writes new RegExp(RegExp.escape('\uHEAD') + RegExp.escape('\uTAIL'), 'x') where \uHEAD\uTAIL encodes that non-BMP whitespace character, the resulting RegExp will match the character instead of being interpreted as whitespace and therefore (in x-mode) ignored
• hex-escape ASCII letters at the start of the string so that new RegExp('\c' + RegExp.escape('Z')) will either be an error (in u- or v-mode RegExps) or match the string '\\cZ' (in other RegExps); this also has the effect that the result cannot combine with a preceding \x or \u when the first character is A-F or a-f. Fixes #66.

[bakkot]

cc @erights @waldemarhorwat @gibson042 Please take a look.

[erights]

How do I see a rendered form of this?

[bakkot]

I've put up a rendering here.

[erights]

Thanks. FWIW LGTM, but I delegate an approval decision to @gibson042 who understands this better than I do.

[erights]

Thanks for the rendering. Cut down the review effort for me by a huge factor.