Don't return `TaffyResult` when `Taffy` methods can't fail.

[gibbz00]

What problem does this solve or what need does it fill?

Reduces the amount of end user unwrap calls.

What solution would you like?

Simply remove the result wrapping for the following Taffy methods:

• new_leaf
• new_with_children
• compute_layout
• new_leaf_with_measure
• remove
• children
• layout
• style
• dirty

[nicoburns]

I think this generally makes sense. One thing I would point out that should probably be discussed before this is implemented is that I believe some of these methods are currently using array indexing to access node data (which will panic if the NodeId is invalid), and it may make sense for these to return errors instead.

Possibly we ought to have both panicking and result-returning variants of these methods?