[util] Add hb-ot-shape-closure tool

[util] Add hb-ot-shape-closure tool

Computes all the glyphs that may be generated given a font and
set of Unicode characters.

The order of the Unicode characters is irrelevant.

Sample output:

behdad:util 0$ ./hb-ot-shape-closure Doulos\ SIL\ Regular.ttf f
f f_f

behdad:util 0$ ./hb-ot-shape-closure Doulos\ SIL\ Regular.ttf i
i

behdad:util 0$ ./hb-ot-shape-closure Doulos\ SIL\ Regular.ttf fi
f f_i f_f_i f_f i

behdad:util 0$ ./hb-ot-shape-closure DroidNaskh-Regular.ttf ب
uni0628 uni0628.init uni0628.medi uni0628.fina

behdad:util 0$ ./hb-ot-shape-closure DroidNaskh-Regular.ttf ا
uni0627 uni0627.fina

behdad:util 0$ ./hb-ot-shape-closure DroidNaskh-Regular.ttf با
uni0627 uni0627.fina uni0628 uni0628.init uni0628.medi uni0628.fina

behdad:util 0$ ./hb-ot-shape-closure DroidNaskh-Regular.ttf با --no-glyph-names
5 6 133 134 135 136