Add Optional Support

[tom-huntington]

A formatter for std::optional as per #1367 (comment)

The code is just copied from barry's slideware where the specifier just get parsed through to the formatter of the underling type.

As such there is no customization of printing of the optional's container for which I originally went with the opinionated Some(...) and None, but then changed to Option(...) and Empty. I'm not very convinced about nullopt when you're trying to study complicated output:

{(50258, 0, None), (50364, 0, None), (538, 0, Some(" by")), (428, 42, Some(" your")), (9827, 100, Some(" spell")), (11, 100, Some(",")), (457, 121, Some(" but")), (4374, 100, Some(" release")), (385, 142, Some(" me")), (490, 156, Some(" from")), (452, 168, Some(" my")), (13543, 177, Some(" bands")), (11, 260, Some(",")), (365, 264, Some(" with")), (264, 268, Some(" the")), (854, 270, Some(" help")), (295, 282, Some(" of")), (428, 290, Some(" your")), (665, 300, Some(" good")), (2377, 312, Some(" hands")), (13, 350, Some(".")), (26214, 374, Some(" Gentle")), (6045, 442, Some(" breath")), (50816, 452, None), (50816, 452, None), (295, 1223, Some(" of")), (6342, 487, Some(" yours")), ...

{(50258, 0, Empty), (50364, 0, Empty), (538, 0, Option(" by")), (428, 42, Option(" your")), (9827, 100, Option(" spell")), (11, 100, Option(",")), (457, 121, Option(" but")), (4374, 100, Option(" release")), (385, 142, Option(" me")), (490, 156, Option(" from")), (452, 168, Option(" my")), (13543, 177, Option(" bands")), (11, 260, Option(",")), (365, 264, Option(" with")), (264, 268, Option(" the")), (854, 270, Option(" help")), (295, 282, Option(" of")), (428, 290, Option(" your")), (665, 300, Option(" good")), (2377, 312, Option(" hands")), (13, 350, Option(".")), (26214, 374, Option(" Gentle")), (6045, 442, Option(" breath")), (50816, 452, Empty), (50816, 452, Empty), (295, 1223, Option(" of")), ...

[tom-huntington]

wchar_t formatting is now working for std::optional, although not for std::variant

[vitaut]

Thanks for the PR. For consistency with other formatters such as variant I suggest replacing Option with optional and use lowercase empty (or none).

[tom-huntington]

Thanks for the PR. For consistency with other formatters such as variant I suggest replacing Option with optional and use lowercase empty (or none).

@vitaut sure, I was just thinking you might, rather, want to change the variant formatter (despite capitalization going against the standard library's convention). For readability.

[vitaut]

Thank you!