masonry: reimplement `Widget::get_child_at_pos` #1484

Job	Run time
cargo check (msrv) (wasm32)	41s
cargo check (aarch64-android)	1m 50s
cargo clippy (windows-latest)	36s
cargo check (msrv) (windows-latest)	1m 35s
cargo clippy (macos-latest)	36s
cargo clippy (wasm32)	1m 3s
cargo check (msrv) (macos-latest)	36s
cargo clippy (ubuntu-latest)	35s
cargo doc	1m 39s
cargo check (msrv) (ubuntu-latest)	35s
formatting	31s
cargo test (wasm32)	19s
Prime LFS Cache	4s
typos	7s
cargo test (windows-latest)	4m 10s
cargo test (macos-latest)	1m 22s
cargo test (ubuntu-latest)	5m 52s
	22m 11s

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