Flatten optionality of regex captures

[hamishknight]

We currently nest optionals for regex literal captures, we should flatten them out to match the regex literal proposal.

[hamishknight]

@milseman do you recall what needs to be done here? Are we also proposing to flatten the dynamic captures of Regex.init (i.e only the DSL should have nested optional captures)?

[milseman]

Yes, we plan to flatten them all outside the DSL. @Azoy can you come up to speed on this? One approach is that we store a isOptional instead of a depth in structured captures representation and the DSL will use the value to store successively wrapped Substring?s. WDYT?