masonry: reimplement `Widget::get_child_at_pos` #1499

Job	Run time
Prime LFS Cache	4s
cargo clippy (wasm32)	54s
cargo check (msrv) (wasm32)	23s
cargo check (msrv) (windows-latest)	1m 58s
cargo clippy (windows-latest)	3m 24s
cargo check (msrv) (macos-latest)	25s
cargo check (aarch64-android)	22s
cargo clippy (macos-latest)	1m 1s
cargo check (msrv) (ubuntu-latest)	47s
cargo clippy (ubuntu-latest)	1m 29s
cargo doc	1m 22s
formatting	32s
cargo test (wasm32)	19s
typos	7s
cargo test (windows-latest)	3m 45s
cargo test (macos-latest)	1m 36s
cargo test (ubuntu-latest)	4m 1s
	22m 29s

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