Revamp type descriptors #310

Job	Run time
rustfmt	1m 35s
static builds (ubuntu-latest, stable)	1m 19s
Address sanitizer	1m 21s
static builds (windows-latest, stable-msvc)	6m 2s
conda (ubuntu, conda-forge, stable)	47s
test apt (stable)	45s
static builds (windows-latest, stable-gnu)	5m 13s
conda (windows, conda-forge, stable)	3m 15s
test apt (beta)	39s
static builds (macos-11, stable)	5m 6s
conda (macos, conda-forge, stable)	1m 0s
test apt (nightly)	40s
static builds (macos-latest, stable)	2m 38s
	30m 20s

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