Convert mutable layout getters to layout setters #1999

Job	Run time
Test Suite [Features: Default]	1m 56s
Test Suite [Features: None]	1m 56s
Test Suite [Features: Default + serde]	2m 1s
Test Suite [Features: Default] (Rust 1.65)	5m 48s
Build Benchmarks	44s
Clippy	31s
Documentation	13s
Build Benchmarks (w/ yoga)	1m 1s
Test Fixture Formatting	15s
Rustfmt	13s
Markdown Lint	3s
Test Suite [Features: alloc]	1m 58s
Test Suite [Features: Default except content_size]	1m 59s
Test Suite [Features: std (no grid)]	2m 9s
Test Suite [Features: flexbox + std]	1m 57s
Test Suite [Features: grid + std]	2m 0s
Test Suite [Features: grid + alloc]	2m 1s
Test Suite [Features: std (no grid or flexbox)]	1m 58s
	28m 43s

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