std.datetime.interval: add Interval.fromISOString for ISO 8601 interv…

changelog/interval_fromisostring.dd

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+Interval, PosInfInterval and NegInfInterval ISO 8601 string support
+
+std.datetime.interval.Interval gained fromISOString, fromISOExtString,
+toISOString and toISOExtString, with PosInfInterval and NegInfInterval
+gaining the same for their open-ended forms. The strings hold two time
+points or a time point and a duration separated by a slash, e.g.
+$(D "20030115/P1M"), where per ISO 8601 an omitted side means that it is
+unknown: Interval then extends to the smallest or largest time point which
+the time point type can represent, whereas PosInfInterval and
+NegInfInterval represent true infinity, e.g. $(D "20030115/") and
+$(D "/20030215").
+
+As with the date and time types in std.datetime, each value in a duration
+must be within the range which its unit can hold (e.g. hours are at most
+23), so a larger span must be expressed with the next larger designator,
+e.g. $(D P1DT12H) rather than $(D PT36H).
+
+Example:
+-------
+import std.datetime.date;
+import std.datetime.interval;
+
+auto i = Interval!Date.fromISOString("20030115/P1M");
+assert(i == Interval!Date(Date(2003, 1, 15), Date(2003, 2, 15)));
+
+// Round trip: parse, write, parse again, and compare.
+assert(Interval!Date.fromISOString(i.toISOString()) == i);
+
+// An omitted side means that the interval extends as far as the time
+// point type can represent.
+assert(Interval!Date.fromISOString("20030115/") ==
+       Interval!Date(Date(2003, 1, 15), Date.max));
+assert(PosInfInterval!Date(Date(2003, 1, 15)).toISOString() == "20030115/");
+-------