Expose hb_tag_from_string

[simoncozens]

This doesn't work:

s = inp.layout_feature_tag_set
s.add("ss01")

Quite rightly, because this is a cdef Set of int. What we are supposed to put in this Set is the output of hb_tag_from_string. I can obviously reimplement this in Python, but it would be helpful if the functionality was exposed from uharfbuzz itself.

[khaledhosny]

So far we have tried to accept strings whenever HarfBuzz wants tags and do the conversation internally. But I guess this is one case where there is no clean way to do this.

[simoncozens]

Maybe an add_tag method on the Set?

[khaledhosny]

That might work.