Implement `box-sizing: content-box` #2245

Job	Run time
Build Benchmarks (w/ yoga)	58s
Build Benchmarks	34s
Documentation	26s
Clippy	32s
Test Fixture Formatting	10s
Rustfmt	10s
Markdown Lint	4s
Test Suite [alloc + all algorithms]	1m 32s
Test Suite [std + block)]	1m 42s
Build [debug feature]	7s
Test Suite [default features]	1m 41s
Test Suite [default except content_size]	1m 26s
Test Suite [default + serde]	1m 32s
Test Suite [std + flexbox]	1m 37s
Test Suite [std + grid]	1m 26s
Test Suite [alloc + no algorithms]	1m 34s
MSRV Build [Rust 1.65]	1m 48s
Test Suite [std (no algorithms)]	1m 47s
Test Suite [no features]	1m 28s
	20m 34s

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