Add functional style regex pattern-matching

Based on Scala's pattern-matching with regular expressions syntax, I created the function `std.regex.switchRegex`. It's main use-case is for reading textual input fields that can be in multiple formats:

```D
    enum WeekDay
    {
        Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday
    }

    struct Schedule
    {
        WeekDay day;
        int hour;
    }

    assert(equal(
                [
                    Schedule(WeekDay.Sunday, 1),
                    Schedule(WeekDay.Monday, 14),
                    Schedule(WeekDay.Tuesday, 3),
                    Schedule(WeekDay.Wednesday, 4),
                    Schedule(WeekDay.Thursday, 17),
                    Schedule(WeekDay.Friday, 6),
                ],
                [
                    "Sunday 1AM",
                    "Monday 2PM",
                    "Tuesday 3",
                    "4AM Wednesday",
                    "5PM Thursday",
                    "6 Friday",
                ].map!(schedule => schedule.switchRegex(
                        `(\w+) (\d+)AM`, (WeekDay day, int hour) => Schedule(day, hour % 12),
                        `(\w+) (\d+)PM`, (WeekDay day, int hour) => Schedule(day, hour % 12 + 12),
                        `(\w+) (\d+)`, (WeekDay day, int hour) => Schedule(day, hour),
                        `(\d+)AM (\w+)`, (int hour, WeekDay day) => Schedule(day, hour % 12),
                        `(\d+)PM (\w+)`, (int hour, WeekDay day) => Schedule(day, hour % 12 + 12),
                        `(\d+) (\w+)`, (int hour, WeekDay day) => Schedule(day, hour),
                        ))())); 
```