Add benchmark test for compound glyph shape

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