Remove out-of-script exemplars

[simoncozens]

This PR fixes a number of issues with the exemplar glyphs. It fixes #24.

• The exemplars for ku_Cyrl and tk_Arab were in the Latin script, not Cyrillic or Latin. These were removed.
• The exemplars for shi_Arab were actually in Tifingah script. These were moved to shi_Tfng.
• bugi_Latin schwa characters were actually Cyrillic, not Latin; these were replaced with Latin schwa characters.
• The exemplars for ccp_Beng were in Chakma, not Bengali. These were removed.

This also includes a test to ensure that the exemplar glyphs are in the script that they are defined to be.

[moyogo]

@simoncozens Should languages have a repeated field auxiliary_script?

Otherwise LGTM.

[simoncozens]

Possibly. There is an interesting case in some Cyrillic languages whereby they do use glyphs from the Latin range as well as Cyrillic glyphs. But I'm not sure if changing the textproto schema is something that is in our power to do.