Switch from Cap'n'proto to Serde for cache #11402

Job	Run time
test-wasm	1m 27s
validate	19s
rustfmt	8s
test (stable, x86_64-unknown-linux-musl)	5m 47s
test (stable, aarch64-unknown-linux-gnu)	3m 46s
test (stable, aarch64-apple-darwin, macos-latest, arm64, cargo, true)	6m 27s
test (stable, x86_64-unknown-linux-gnu)	4m 41s
test (stable, x86_64-pc-windows-msvc)	5m 54s
test (stable, aarch64-unknown-linux-musl)	4m 40s
lint-build	1m 4s
test (stable, x86_64-apple-darwin)	13m 5s
test-projects	2m 33s
	49m 51s

Provide feedback