Metadata ergonomics

[molpopgen]

The current metadata API returns Result<Option<_>, _>. The error state happens when inputs are out of bounds. We should take a clue here from stdlib and just return None for such out of bounds cases.

IMO, it is worth it to streamline code using this part of the API.

[molpopgen]

There are 2 sources of error:

• range errors.
• decoding errors.

The latter is the main ergonomic issue.
Here, it may be that the best thing to do is to recommend using Result::ok.

[molpopgen]

Closing this -- the entire lib treats out of range as an error and we still have to worry about serde, etc., failing. No sense in breaking API for no reason.

[molpopgen]

Reopening -- it'd be nice to be more idiomatic w/respect to indexes not in range.