wip: refactoring extension handling #696

Job	Run time
Validate external types appearing in public API	1m 12s
Format	12s
Documentation (stable)	30s
build (macOS-latest, stable)	46s
Documentation (beta)	26s
build (ubuntu-latest, stable)	4m 57s
Documentation (nightly)	30s
build-windows	7m 24s
build (ubuntu-latest, beta)	32s
Documentation (stable 7 months ago)	33s
Clippy	4m 40s
build (ubuntu-latest, nightly)	5m 14s
Measure coverage	41s
build (ubuntu-latest, stable 7 months ago)	38s
Check semver compatibility	25s
	28m 40s

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