Rename WeightedError → WeightError; add IndexedRandom, IndexedMutRandom #655

Job	Run time
test-ios	31s
Check doc	1m 6s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 31s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 16s
test (macos-latest, x86_64-apple-darwin, stable)	1m 34s
test-miri	2m 48s
test-no-std	14s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 21s
test (windows-latest, x86_64-pc-windows-msvc, beta)	2m 0s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.60.0)	4m 11s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	41s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	31s
	18m 44s

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