Safe rework of get_glyph_shape

The change aims to address shortcomings in the get_glyph_shape implementation

• unsafe usage for vec initialization
• cyclomatic_complexity warning - function is too big

And solve them without sacrificing too much performance.

I addressed the complexity issue by splitting out the number_of_contours > 1 case into another private method. This method is #[inline]ed to eliminate performance impact.

The unsafe set_len(m) initialization was trickier. A simple safe initialization with a dummy Vertex provided a drop in solution. But performed noticeably worse.

vec![dummy; m] init

name                          control ns/iter  change ns/iter  diff ns/iter  diff %  speedup 
get_glyph_shape_deja_vu_mono  12,949           15,024                 2,075  16.02%   x 0.86 
get_glyph_shape_gudea         11,407           14,336                 2,929  25.68%   x 0.80

After staring at the algorithm for a bit I found it a little odd. The first thing it does is find "flag" data for the vertices and saves it as Vertex data at the end of the vec. However, it isn't vertex data, it is simply used to compute the actual vertex data which is added from the beginning of the vec. The real Vertexs then flow over the flag data, but in real usage the read flag data will always be ahead. To me this is all a bit weird.

I separated the flag data into it's own vec, which removes the need to add fake-vertex structs to the back end of the vec. It also separates something that is separate, allows for FlagData to be a smaller struct than Vertex with specialized types.

Adding in some other minor type changes and byte reading improvements and we have a safe version of the algo, without too much performance loss.

rework

name                          control ns/iter  change ns/iter  diff ns/iter  diff %  speedup 
get_glyph_shape_deja_vu_mono  12,949           12,861                   -88  -0.68%   x 1.01 
get_glyph_shape_gudea         11,407           11,901                   494   4.33%   x 0.96 

Resolves #15 (closed), #16 (closed)