masonry: reimplement `Widget::get_child_at_pos` #1616

Job	Run time
cargo check (msrv) (wasm32)	16s
cargo clippy (windows-latest)	2m 34s
cargo check (aarch64-android)	19s
cargo check (msrv) (windows-latest)	2m 40s
cargo clippy (macos-latest)	1m 35s
cargo check (msrv) (macos-latest)	24s
cargo clippy (wasm32)	1m 17s
cargo clippy (ubuntu-latest)	1m 43s
cargo doc	1m 29s
cargo check (msrv) (ubuntu-latest)	39s
formatting	37s
Prime LFS Cache	4s
cargo test (wasm32)	23s
typos	6s
cargo test (windows-latest)	3m 0s
cargo test (macos-latest)	1m 24s
cargo test (ubuntu-latest)	3m 40s
	22m 10s

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