Remove u64 support for Poisson (#1517) #1116

Job	Run time
Check doc	34s
test-miri	4m 28s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	18m 16s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	3m 5s
test (macos-latest, x86_64-apple-darwin, stable)	2m 49s
test (windows-latest, x86_64-pc-windows-gnu, stable)	4m 24s
test (windows-latest, x86_64-pc-windows-msvc, beta)	4m 1s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	5m 6s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	4m 42s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	3m 37s
test-ios	27s
test-no-std	14s
	51m 43s

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