Add SomeTableProvider trait for traversal and use it in otexplorer

[valadaptive]

The get_some_table function was getting a bit out-of-date (it was last updated 3 years ago). I feel like it's tied more to read-fonts than otexplorer, since it needs to know all of the tables that the TableProvider trait is capable of returning. As such, I've created a SomeTableProvider trait in read-fonts, which has one method that does what get_some_table used to do.

I read through the issues a bit and it seems like the traversal API might get removed at some point, but some people find otexplorer very useful for debugging. I concur; I've found myself using it a lot to look inside fonts.