Address Str4d’s comments on #171 (#175)

* Address Str4d’s comments on #171

Notably, `HashType` has changed incompatibly, so
ZcashFoundation/zebra#8751 will need to be updated.

* Apply suggestions from code review

Co-authored-by: Jack Grigg <[email protected]>

* Restrict bitflags used for `HashType` in v5 tx

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Co-authored-by: Jack Grigg <[email protected]>

Author: sellout