use `NonZeroU8` for PDA seeds

[billythedummy]

https://github.com/anza-xyz/solana-sdk/blob/5676afef789520cf4593e048e9929d1f5b23e479/address/src/syscalls.rs#L308

0 is not a valid bump seed.

Edit: maybe no for now, since it breaks current API and API compat with upstream