Review API for type improvements

[madig]

See https://blog.logrocket.com/improving-overconstrained-rust-library-apis/, e.g. the "Into the Woods" section for possible improvements to functions taking optional arguments.

[cmyr]

we do similar things in a few places, like various arguments take impl Into<Arc<T>>, so you can pass either an existing Arc<T> or just a T.

[madig]

Ah, yes. I'm thinking GlyphBuilder and the various new functions for anchors, etc.