Add straight spur variant for lowercase a

[nathanielevan]

These patches may replace the default, curved-spur lowercase 'a' shape (and its diacritical variants) of Hack v2+ with a straight-spur variant found in version 1 of Hack (and the default 'a' shape in DejaVu Sans Mono).

Hack v3.003+	Straight spur patch

Admittedly a minor design alteration, this patch has a side effect of fixing the bloated 'a' bowl that ttfautohint produces at size 12ppem (9pt at 96dpi) with the default curved-spur variant. Manual hinting should take care of this issue, but as far as I understand it, using the hintslight setting on fontconfig implicitly uses freetype's autohinter [1][2] hence foregoing the font's built-in hinting instructions. Here's a comparison of the hinting result at 12ppem:

Hack v3.003+	Straight spur patch

I don't have access to Glyphs.app, however—this patch was hand-typed in a text editor and previewed in FontForge, hence its results may be rudimentary and need further fine-tuning. Let me know what you think, either way.