Prepare 0.6.0-beta.1 release #160

Job	Run time
fmt	26s
build (stable, --features "")	1m 49s
clippy	30s
test	1m 45s
build (stable, --all-features)	2m 54s
build (stable, --no-default-features)	1m 37s
build (1.80, --features "")	1m 33s
build (1.80, --all-features)	3m 19s
build (1.80, --no-default-features)	2m 25s
build (beta, --features "")	1m 20s
build (beta, --all-features)	2m 55s
build (beta, --no-default-features)	1m 32s
build (nightly, --features "")	2m 19s
build (nightly, --all-features)	1m 40s
build (nightly, --no-default-features)	1m 22s
	27m 26s

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