Addresses the following review comment:
- "I think this should be a constructor `AccountId::for_private_pda`.
Consider also removing the existing `impl From<(ProgramId, Seed)> for
AccountId` for public pdas in favor of a `AccountId::for_public_pda`
to have a unified way of constructing pdas"
I replaced `impl From<(&ProgramId, &PdaSeed)> for AccountId` with
`AccountId::for_public_pda(program_id: &ProgramId, seed: &PdaSeed) ->
Self` and replaced the free function `private_pda_account_id(...)`
with `AccountId::for_private_pda(program_id: &ProgramId, seed:
&PdaSeed, npk: &NullifierPublicKey) -> Self`. Both live in an inherent
`impl AccountId` block in nssa/core/src/program.rs next to the PDA
derivation logic. Migrated all call sites across nssa/core,
nssa/src/state.rs, nssa/src/validated_state_diff.rs,
program_methods/guest/src/bin/privacy_preserving_circuit.rs,
programs/amm/core, programs/associated_token_account/core, the example
tail-call binary, and the ATA tutorial doc. Test function names that
referenced the old free function were also renamed
(private_pda_account_id_* to for_private_pda_*).
