Fix Pcre2.exec_all for non-consuming patterns

[Andrew-William-Smith]

At present, Pcre2.exec_all does not return the full set of captures when matching patterns containing non-consuming captured subpatterns such as lookahead assertions, falsely detecting the end of the matched set before reaching the end of the string. In order to resolve this issue, I have implemented the algorithm recommended by the PCRE2 project for global regex matching, allowing for zero-length captures to be properly matched and returned by exec_all.

As a demonstration, prior to this patch:

# Pcre2.exec_all ~pat:"(?=(hello|ocaml))" "hellocaml" |> Array.map Pcre2.get_substrings;;
[|[|""; "hello"|]|]

And after this patch:

# Pcre2.exec_all ~pat:"(?=(hello|ocaml))" "hellocaml" |> Array.map Pcre2.get_substrings;;
[|[|""; "hello"|]; [|""; "ocaml"|]|]

Additionally, the test suite has been expanded to cover a number of evaluation scenarios for exec_all, including zero-length matches such as those shown above.

[Andrew-William-Smith]

For what it's worth, I now have a branch on my fork that supersedes this patch and adds Seq.t variants of all functions that perform multiple matches (exec, extract, split): if you have an interest in merging that functionality, then I'm happy to close this PR in favour of a prospective new one that would include the added sequence functions.