optimize PL deserialization #2

Job	Run time
xtask	48s
Docs	1m 55s
Minimum Supported Rust Version / Check All Features	2m 0s
Rust Nightly / Check Formatting	25s
Rust Stable / Check All Features	1m 52s
Minimum Supported Rust Version / Check Ruma	51s
Rust Nightly / Full Features	48s
Style	48s
Rust Stable / Check Client	52s
Minimum Supported Rust Version / Check Owned IDs with Box	55s
Rust Nightly / All Features	54s
Rust Stable / Check Common	49s
Minimum Supported Rust Version / Check Owned IDs with Arc	50s
Rust Nightly / Clippy Default Features	39s
Rust Stable / Run Tests	1m 48s
Rust Nightly / Clippy All Features	44s
Rust Stable / Run Compat Tests	1m 49s
Rust Nightly / Clippy WASM	40s
Rust Stable / Run Doc Tests	1m 32s
	20m 59s

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