Index all homs in a schema when using `@acset_type`

[slwu89]

Programmer QoL feature, it would be nice to do something like @acset_type MyType(MySch, index=true) to indicate that all homs in the schema should be indexed.

EDIT: not sure how necessary this is, as one can do @acset_type MyType(MySch, index=nameof.(generators(MySch,:Hom)))

[epatters]

I think it would still be a nice convenience feature, as it comes up fairly often. PR welcome.