Add benchmark test for compound glyph shape

Merged Alex Butler requested to merge (removed):bench-compound-glyph-shape into master

Add regression benchmarking & optimisations for compound glyphs. Uses roboto-regular ':'.

With the regression test in place I made some minor byteorder read optimisations.

 name                                         control ns/iter  change ns/iter  diff ns/iter  diff %  speedup 
 get_glyph_shape_compound_glyph_roboto_colon  460              438                      -22  -4.78%   x 1.05 

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