Fira 3.108 hacked

This replaces the Fira 2.001 fonts with 3.108 (actually, some of the Fira Sans faces are marked 3.107, but I guess that's because they weren't changed between the two releases). The Fira Sans italic faces where some glyphs had negative advance widths have been hacked such that the glyphs are now zero-width instead, to avoid potentially confusing layout.

Note that this is untested as yet!