feat: tower should be an optional dependency #249

Job	Run time
Check Style	4s
features	32s
Test with Miri	41s
Check MSRV (1.63)	1m 12s
Test stable on ubuntu-latest	27s
Test stable on windows-latest	1m 48s
Test stable on macOS-latest	45s
Test beta on ubuntu-latest	29s
Test beta on windows-latest	1m 5s
Test beta on macOS-latest	56s
Test nightly on ubuntu-latest	31s
Test nightly on windows-latest	1m 4s
Test nightly on macOS-latest	47s
Build docs	19s
CI is green	0s
	10m 40s

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