* refactor: use auto-generated HashTreeRoot functions in htrutil.go
* refactor: use type alias for Transaction & use SliceRoot for TransactionsRoot
* changelog
* fix: TransactionsRoot receives raw 2d bytes as an argument
* fix: handle nil argument
* test: add nil test for fork and checkpoint
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Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>