masonry: reimplement `Widget::get_child_at_pos` #1617

Job	Run time
cargo check (aarch64-android)	24s
cargo check (msrv) (windows-latest)	2m 21s
cargo check (msrv) (wasm32)	28s
cargo clippy (windows-latest)	3m 23s
cargo check (msrv) (macos-latest)	26s
cargo clippy (macos-latest)	1m 25s
cargo clippy (wasm32)	1m 9s
cargo check (msrv) (ubuntu-latest)	35s
cargo clippy (ubuntu-latest)	1m 44s
cargo doc	1m 31s
formatting	33s
Prime LFS Cache	9s
cargo test (wasm32)	25s
typos	5s
cargo test (windows-latest)	3m 21s
cargo test (macos-latest)	1m 27s
cargo test (ubuntu-latest)	4m 11s
	23m 37s

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