fix(daysUntil): use larger integer type for proper underflow

[rockorager]

Use a larger integer type for proper underflow in Weekday.daysUntil.

This is because the algorithm assumes a larger integer than a u3.
It's relying on underflow to work:

https://howardhinnant.github.io/date_algorithms.html#weekday_difference

For Wednesday -> Monday, the math is:

Monday (1) - Wednesday (3) = -2

-2 as a u3 = 6, but we are checking if the difference is <= 6 so we
return 6 instead of adding 7.

If we do this as a u8, -2 is 254. Add 7 with overflow, and we get 5.

Note that this exposed a bug in the DST calculation for fixed posix
timezones as well. The days var suffered from an off-by-one bug. It is
supposed to be the day of the month (1-31), NOT a zero-indexed day.

Fixes: #8