masonry: reimplement `Widget::get_child_at_pos` #1498

Job	Run time
cargo clippy (wasm32)	56s
cargo clippy (windows-latest)	2m 33s
cargo clippy (macos-latest)	1m 2s
cargo check (msrv) (windows-latest)	1m 47s
cargo check (msrv) (wasm32)	17s
cargo check (msrv) (macos-latest)	26s
cargo clippy (ubuntu-latest)	1m 31s
cargo check (aarch64-android)	24s
cargo check (msrv) (ubuntu-latest)	42s
cargo doc	1m 28s
formatting	42s
cargo test (wasm32)	22s
Prime LFS Cache	4s
typos	4s
cargo test (windows-latest)	3m 18s
cargo test (macos-latest)	1m 20s
cargo test (ubuntu-latest)	4m 12s
	21m 8s

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