Restructure download script to avoid exceedingly many parallel fetches

Ref. #6.
Download Unicode Bidi(rectionality) property files

These properties support the Unicode Bidirectional Algorithm.

Ref. #6.
Add Bidi properties as new Bidi_* categories

Ref. #6.
Add BidiMirroring tables

We also export an array directly mapping code points to properties where that makes sense: canonical category, bidi property, and the mirroring glyph.

Ref. #6.
Add BidiBrackets tables

Ref. #6.
Update dependencies
Minor cleanup