Refactor the host <-> runtime interface machinery #20183

Job	Run time
check-dependency-rules	12s
check-workspace	16s
check-fail-ci	20s
check-markdown	20s
check-readme	3m 7s
preflight / preflight	51s
preflight / ci-versions	56s
check-rust-feature-propagation	2m 2s
test-rust-features	1m 52s
fmt	1m 10s
check-umbrella	2m 0s
check-toml-format	1m 3s
All quick checks passed	1s
	14m 10s

Provide feedback