chore: Move CosetIndex and CommitmentIndex into kzg_multi_open #248

Job	Run time
Build - x86_64-unknown-linux-gnu	1m 11s
Build - aarch64-unknown-linux-gnu	1m 11s
Build - aarch64-apple-darwin	1m 7s
Build - x86_64-apple-darwin	1m 12s
Build - x86_64-pc-windows-gnu	4m 4s
Test - x86_64-unknown-linux-gnu	53s
Test - aarch64-unknown-linux-gnu	1m 30s
Test - x86_64-apple-darwin	57s
Test - aarch64-apple-darwin	59s
Test - x86_64-pc-windows-gnu	1m 33s
Publish	0s
	14m 37s

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