[designspace] Stop generating source/instance names

[cmyr]

closes #312

[Hoolean]

Thanks for putting this PR together Colin, you beat me to it! Nice to see that only minimal changes are needed in fontc too 👍

[RickyDaMa]

Looks good to me, if this gets merged I'll rebase my PR over it :)

[anthrotype]

Nice to see that only minimal changes are needed in fontc too

fontc relies on those name attributes being set to non Optional values (e.g. it uses the source.name as a unique way to identify specific source elements, for the filename may well be the same for multiple sources referencing different layers from the same .ufo), so this is a breaking change.. If norad is unwilling to provide these unique name attributes fontc will have to do the same dance with AtomicU64. It's not sufficient to fallback to a constant "placeholder_source_name" because that might well clash.

[cmyr]

I have a patch for fontc written, and I do think that this belongs there, instead of norad.