Bidi reordering engine upgraded

[Eli Zaretskii]

With revision 118121, the upgraded bidi reordering engine landed on
the trunk.  Barring bugs, we are now fully compliant with The Unicode
Bidirectional Algorithm defined by the latest Unicode Standard v7.0,
and even with a few suggestions that will only make it into Unicode
8.0.

The changes in the UBA as defined by the latest Unicode Standard, wrt
to the original UBA we had until and including Unicode 6.2, are
significant.  They required serious refactoring and reimplementation
of several core parts in bidi.c, something that wasn't attempted for
quite a few years.

In addition, one of the new UBA features, the so-called Bidirectional
Parentheses Algorithm (BPA), affects pure-ASCII text as well, and
specifically editing of program sources (which widely use parentheses
and brackets of several kinds).

The result is some small slowdown -- a few percents in my testing --
in redisplay operations.  If more significant slowdown will be
reported in some special cases, I will try to find optimizations to
countermand that.

No matter how much testing I've put into the new code, there will
probably be bugs.  Please be attentive to any display glitches in the
trunk code, let alone crashes etc., and report them as bugs.

Thanks, and enjoy.