Prepare for v0.4.0 release #17

Job	Run time
Clippy (ubuntu-latest, stable)	28s
Build (ubuntu-latest, stable)	24s
Clippy (ubuntu-latest, nightly)	26s
Test (ubuntu-latest, stable)	22s
Build (ubuntu-latest, 1.56.0)	3m 45s
Test (ubuntu-latest, 1.56.0)	3m 51s
Build (macOS-latest, stable)	1m 2s
Test (macOS-latest, stable)	59s
Build (macOS-latest, 1.56.0)	4m 33s
Test (macOS-latest, 1.56.0)	8m 47s
Build (windows-latest, stable)	2m 3s
Test (windows-latest, stable)	2m 3s
Build (windows-latest, 1.56.0)	6m 4s
Test (windows-latest, 1.56.0)	6m 43s
Format (ubuntu-latest, stable)	9s
Format (ubuntu-latest, nightly)	18s
	41m 57s

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