Previously, each code point's General Category was part of the generated
UnicodeData structure. This ultimately presented two problems, one
functional and one performance related:
* Some General Categories are applied to unassigned code points, for
example the Unassigned (Cn) category. Unassigned code points are
strictly excluded from UnicodeData.txt, so by relying on that file,
the generator is unable to handle these categories.
* Lookups for General Categories are slower when searching through the
large UnicodeData hash map. Even though lookups are O(1), the hash
function turned out to be slower than binary searching through a
category-specific table.
So, now a table is generated for each General Category. When querying a
code point for a category, a binary search is done on each code point
range in that category's table to check if code point has that category.
Further, General Categories are now parsed from the UCD file
DerivedGeneralCategory.txt. This file is a normal "prop list" file and
contains the categories for unassigned code points.
There are a couple of minor nuances with parsing script values, compared
to other properties. In Scripts.txt, the UCD file lists the full name of
each script; other properties, like General Category, list the shorter
name in their primary files. This means that the aliases listed in
PropertyValueAliases.txt are reversed for script values.
This downloads the PropertyValueAliases.txt UCD file, which contains a
set of General Category aliases.
This changes the General Category enumeration to now be generated as a
bitmask. This is to easily allow General Category unions. For example,
the LC (Cased_Letter) category is the union of the Ll, Lu, and Lt
categories.
Adds methods to retrieve a Unicode property from a string and to check
if a code point matches a Unicode property.
Also adds a <LibUnicode/Forward.h> header.
