Add Guard::into_inner_if_some

[asonix]

Allows passing the Key slice to the predicate and receiving a parsed value, returning that parsed value as the first element of the returned tuple. This can avoid double-parsing of keys that might happen with just Guard::into_inner_if

[asonix]

resolve_value_handle needs access to the key slice, so the predicate can't take ownership of it.

[codecov]

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[marvin-j97]

    ) -> crate::Result<(K, Option<UserValue>)> {

would probably be the more flexible API as it allows reusing the parsed key even if a value was chosen not to be loaded.

[asonix]

@marvin-j97 could you explain how that would work? if the predicate returns None rather than Some(K), how could we then return a K from the function?

[marvin-j97]

Right, no, I'm stupid - it would need a different signature.

[asonix]

We could return a Result<Result<(K, UserValue), UserKey)>> maybe, so if the K can't parse the unparsed key can still be accessed

I also think we don't care about this function returning a key without loading the value. If a user wants to do that they can simply call guard.key() and then parse that key.

[asonix]

Changed to return Result<Result<(K, UserValue), UserKey>>, so the unparsed key can be made available for inspection