Fix assume timezone

[Yuri6037]

This PR finally fixes the behavior of assume_timezone and assume_timezone_utc. The assume_timezone function is now fixed to always assume the input PrimitiveDateTime is already in the target timezone. The assume_timezone_utc preserves the old behavior of assuming the input PrimitiveDateTime is actually in UTC.

Fixes #5
Fixes #6

[korrat]

FYI, if you want to prevent traits like PrimitiveDateTimeExt from being implemented outside your crate, you can seal them with a trick. Basically, you add a trait to a private module in your crate and use it as a trait bound on your public traits. Since the trait in the private module is not visible outside your crate, nobody can implement it outside your crate. Therefore, outside your crate, nobody can implement the public traits either. The Rust API Guidelines have a bit more details on that.

[Yuri6037]

FYI, if you want to prevent traits like PrimitiveDateTimeExt from being implemented outside your crate, you can seal them with a trick. Basically, you add a trait to a private module in your crate and use it as a trait bound on your public traits. Since the trait in the private module is not visible outside your crate, nobody can implement it outside your crate. Therefore, outside your crate, nobody can implement the public traits either. The Rust API Guidelines have a bit more details on that.

Wow thanks a lot I had no idea this existed. I'll try looking into this.

[Yuri6037]

I've applied the new API changes with the sealed trait trick. I've also included a feature to allow removing the tz database with this 1.0 release. The feature is there in case you wanted to use your own custom timezone database or maybe even use the system database instead of the built-in one.