feat(engine): Optional Serde #3940

Job	Run time
doctest	1m 41s
test ubuntu-latest stable --no-default-features	1m 31s
test ubuntu-latest stable	1m 33s
cfg-check	39s
check no_std	23s
clippy	51s
test ubuntu-latest stable --all-features	1m 45s
test ubuntu-latest nightly --no-default-features	1m 40s
docs	1m 3s
feature-checks	2m 6s
wasm-unknown	57s
test ubuntu-latest nightly	1m 34s
wasm-wasi	1m 59s
test ubuntu-latest nightly --all-features	1m 45s
test ubuntu-latest 1.79 --no-default-features	44s
fmt	14s
test ubuntu-latest 1.79	50s
test windows-latest stable --no-default-features	1m 14s
test windows-latest stable	2m 30s
test windows-latest stable --all-features	3m 5s
test windows-latest nightly --no-default-features	1m 45s
test windows-latest nightly	2m 3s
test windows-latest nightly --all-features	1m 15s
test windows-latest 1.79 --no-default-features	2m 15s
test windows-latest 1.79	1m 57s
	37m 19s

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