Add reset, find_opt and Seq functions to BatHashtbl #61

Job	Run time
build-dune (ubuntu-latest, 5.1.0, true)	4m 18s
build-dune (ubuntu-latest, 5.0.0, true)	3m 21s
build-dune (ubuntu-latest, 4.14.x, true)	3m 49s
build-dune (ubuntu-latest, 4.13.x, true)	3m 34s
build-dune (ubuntu-latest, 4.12.x, true)	3m 16s
build-dune (ubuntu-latest, 4.11.x, true)	3m 55s
build-dune (ubuntu-latest, 4.10.x, true)	3m 35s
build-dune (ubuntu-latest, 4.09.x, true)	3m 27s
build-dune (ubuntu-latest, 4.08.x, true)	3m 37s
build-dune (ubuntu-latest, 4.07.x, true)	5m 52s
build-dune (ubuntu-latest, 4.06.x, true)	5m 34s
build-dune (ubuntu-latest, 4.05.x, true)	6m 29s
build-dune (ubuntu-latest, 4.02.3, false)	5m 29s
build-dune (macos-latest, 4.14.x, true)	3m 38s
build-dune (macos-latest, 5.0.0, true)	3m 43s
	1h 3m 37s

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