Avoid use of Vec in ZSA-compatible version of lib.rs

[dmidem]

Problem

The current CI build for this zcash_note_encryption crate is failing when attempting to merge ZSA-related changes made in lib.rs. This issue arises from the use of Vec generic defined in the standard library ("std"), while the crate is configured with the #![no_std] directive in lib.rs.

Current solution

To address the CI build failure, a temporary solution has been implemented by modifying the GitHub Actions CI configuration (ci.yml) to enable the "alloc" feature. Since "alloc" also contains the definition of Vec and can serve as a lightweight alternative to "std," this adjustment allows the CI to complete successfully.

ci.yml:

...
        run: cargo build --features=alloc --no-default-features --verbose --target ${{ matrix.target }}
...

While this approach resolves the immediate CI issue, it introduces additional dependencies. This may not be optimal, especially for scenarios involving the usage of zcash_note_encryption in embedded solutions or on the browser side.

Proposed slternative solution

An alternative solution is presented in this draft pull request, involving modifications to the lib.rs file to eliminate the use of Vec. It's important to note that this alternative solution requires corresponding adjustments in the orchard_zsa crate, which implements the Domain trait from zcash_note_encryption.