Interval construction by "earlier" or "later" durations

[erikc5000]

When constructing intervals, it would be nice to support a syntax like:

for (date in today until 1.weeks.later) {
    // ...
}

for (date in 1.weeks.earlier until today) {
    // ...
}

It reads a bit nicer than:

for (date in today until today + 1.weeks) {
    // ...
}

for (date in today - 1.weeks until today) {
    // ...
}

The ".earlier" and ".later" would just be syntactic sugar around a duration unit, but at no added expense if implemented using an inline class. We could forego the the sugar and use the durations directly, but it's a little less clear.

for (date in today until 1.weeks) {
    // ...
}

for (date in 1.weeks until today) {
    // ...
}