Use ok_or_else instead of ok_or in serde decoding

[jrose-signal]

Serde errors are not simple enums; they format a full error string from their arguments. It's worth not doing that up front.

[jrose-signal]

Bumping this, it's free performance! (Also caught by cargo clippy --all-features, in addition to a less-interesting suggestion to use enumerate() for deserializing the 32-tuples representing scalars and points.)

[tarcieri]

Thanks!