Optimize memory usage of data structures

[mwcampbell]

The structures defined in the schema scrate haven't yet been optimized for memory usage. In particular, the Node struct is ~1.5KB on a 64-bit machine.

[i509VCB]

From running cargo +nightly rustc -p accesskit -- -Zprint-type-sizes, this is a breakdown of the size of Node:

https://gist.github.com/i509VCB/c15dfb11b3804cdbd82a28b5e90b3df8

[mwcampbell]

Resolved by #205