DST handling: duplicates and skipped entries traversing reoccurances

[mnahkies]

In a similar vein to #109 we're experiencing some issues with zoned rrule's and daylight saving boundaries.

Specifically:

• When the clocks go forward, we see duplicate entries and one missing entry
  • Edit: the "missing" entry was actually supposed to be missing based on the rrule I used 🤦
• When the clocks go backwards, we see missing entries

I've created a minimal reproduction here that asserts a 4 hour window to two DST boundaries for the Europe/London timezone:
https://github.com/mnahkies/rust-rrule-dst-bug/blob/main/src/test.rs

I'm open to attempting a patch for this, but unsure when I'll capacity to do so.

It would be great to get some feedback whether this would likely to be accepted, or if I'm doing something silly and this isn't an issue to start with.

[mnahkies]

Re-tested with 0.13.0 that was recently released, and still appears to be an issue.

mnahkies/rust-rrule-dst-bug@2862dca