Rrule

Changed

- `RRule` represents only RRULE in the iCalendar specification. And `RRuleSet` is for the whole iCalendar string.
- `RRule` has two stages, `Unvalidated` and `Validated`. When you initialize it, it is `Unvalidated` and by calling the `validate` method, it will change to `Validated`.
- All fields of `RRule` and `RRuleSet` are private, instead there are a set of new methods to get and set values of fields on both structs. It's been done to have a safer API.
- `tz` is removed from all structs. It was a duplicated copy of the timezone inside the `dt_start`. Instead, you can control it by having a proper timezone for [`RRuleSet::dt_start`].

Removed

- Removed `dt_start` and `dt_end` from `RRule`.
- There is no `Default` for `RRuleSet`, instead use `new(UTC.ymd(1970, 1, 1).and_hms(0, 0, 0))` method to have the same behavior.
- Removed `RRuleProperties`, instead you can use `RRule<Unvalidated>`.
- `DateFilter` trait has been removed, since now we can only iterate over `RRuleSet` and the methods are inside `RRuleSet` itself.
- The iterator over `RRule` is not public anymore.

Added

- There is a new `serde` feature to enable serialization and deserialization on `RRuleProperties` and `RRule`.
- `RRuleProperties` and `RRule` structs now implement `Display` and `FromStr` to convert from and to strings.

Changed

- `dt_start` and `tz` moved from `RRuleProperties` to `RRule`

Fixed

- Fix [33](https://github.com/fmeringdal/rust-rrule/issues/33)
- Fix [34](https://github.com/fmeringdal/rust-rrule/issues/34)
- Many tests were ignored because they were either invalid or the test didn't pass. Most ignored tests are now deleted, or the code is fixed so that they pass.
- Better error handling

Changed

Internal refactorings:

- Added `ParseError` and `ValidationError` which contains more specific errors during the parsing and validation phase respectively.

Fixed

- Fix [48](https://github.com/fmeringdal/rust-rrule/issues/48)

Added

- Added `forbid(unsafe_code)` into repo. We don't ever need unsafe code.
- Added additional testing crates: `rrule-debugger` and `rrule-afl-fuzz`.
(only used internally and for debugging)
- Added `examples` folder and moved some code from documentation into examples.
- New and improved error handling consolidated into one object, `RRuleError`.
- Added new trait `DateFilter` for implementing methods like:
`all_with_error`, `all_before_with_error`, `all_after_with_error` and `all_between_with_error` and moved/added methods like `all`, `just_before`, etc.
- Added arbitrary limits for safety reasons.
See [ReadMe](README.mdvalidation_limits) for more info.
- Improved `rrule` command line tool.
- New `WithError` trait, implemented for `RRuleIter` and `RRuleSetIter`.

Changed

- License change, from MIT to (MIT or Apache 2.0). (27)
- Massive code restructuring. Split into `core`, `iter`, `parser` and `validator`.
- Renamed `Frequenzy` to `Frequency`.
- Add limit to `all()`, prevent infinite loops.
- `ParsedOptions` and `Options` are merged together into `RRuleProperties`. (22)
- `Options` functions changed, for example `byminute` -> `by_minute`.
- `by_easter` is now opt-in with feature flag `by-easter`. (26)
- `RRule` can only be crated using `new` function with a valid `RRuleProperties`.
- `RRule.option` is no longer public, but can be read by using `get_properties()`.
- `RRuleIter` and `RRuleSetIter` are now part of the public API.
- `NWeekday` has totally changed but serves the same purpose.
- Updated `chrono-tz` from `0.5.3` to `0.6.0`.
- Function `all` was moved to `DateFilter` and returns a `Result` now.

Deprecated

Removed

- `RRuleSet::new()` replaced with `RRuleSet::default()`.
- `ParsedOptions` and `Options` are now combined as `RRuleProperties`. (22)
- `NWeekdayIdentifier` replaced with new of `NWeekday`.
- `by_n_weekday` field removed from `ParsedOptions`, combined into `by_weekday`.
- `concat` and `build` in `Options` are removed, no longer needed.
- Removed `serde` dependency. (21)
- Removed function `between`, use `DateFilter::all_between` instead.
- Removed function `after`, use `DateFilter::just_after` instead.
- Removed function `before`, use `DateFilter::just_before` instead.

Fixed

- Replaced panic on incorrect datetime with error.
- Fix timezone conversions when `TZID` and `Z` is present.
- Stabilized `RRule` parsing from string. Fixed a lot of possible panics.

Security

- Added security notice to [README.md](README.md).
- The validation of `RRule` improved the stability of the crate a lot.

Pre 0.6.0 (2021-07-02)

All changes before 2021-07-02 where not documented.
This is everything before and including fa8308944a4d2ead0a6ccfa6ee53b76b399e045f