refactor!: Unbox the text selection property

[mwcampbell]

TextSelection used to be much bigger when NodeId was 128-bit. But now we can unbox that property without increasing the total size of PropertyValue. So this PR does that. Also, I decided that the rect accessor should return a reference to the Rect struct rather than a copy. This makes the accessors for large structs more consistent.

[mwcampbell]

I no longer think I want to do this. Instead, I'm thinking about moving the rect property to a direct field on Node. That would increase the size of Node by 32 bytes, but would decrease the size of PropertyValue by 8 bytes, from 40 to 32. That would allow property values to be a little more densely packed, with two property values fitting in a typical 64-byte cache line. And nearly every node has a bounding rectangle, so changing that property into a direct field would eliminate a slot in the Vec of dynamic property values for most nodes, meaning that the initial allocation for that Vec would go a little further.