WIP: Bump up librustzcash to zcash v4.2

This MR is part of the meta issue #1101 (closed) (JavaScript version of Sapling). Please refer to it to see if it does not depend on other MR's first).

Context

Built on top of !2560 (closed), partially !2539 (merged) (need a rebase). TODO

Manually testing the MR

TODO

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• Add item in the Development Version section of CHANGES.md (only for new features and bug fixes).

Reviewers

Propose suitable reviewers for this MR.