Consider switching Taffy API to use interior mutability

[nicoburns]

Currently the LayoutPartialTree trait uses &mut self references when it needs write access to the tree (for writing cache entries and final layout values. This makes sense, however it a couple of downsides:

• It makes it much harder for Taffy to use borrowed references to styles. We can only borrow between writes, and any time a write occurs we have to drop the reference and then reborrow even though in practice the style is never mutated by the cache write.
• This API may not work for parallel layout if we ever implement multithreading.

We should investigate switching to using &self for all trait methods (which would imply the use of interior mutability on the implementors side). This implies a runtime cost but this may well end up not being significant (to be measured).