Replace chrono with time

[olivereanderson]

Description of change

This PR removes all our direct dependencies on the chrono crate and introduces a dependency on time instead.

The changes introduced here are mainly visible internally in identity-core as I have tried to keep the changes to the Timestamp API to a minimum.

Links to any relevant issues

fixes issue #442

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Runs under cargo test

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[cycraig]

Thanks for the change.

parse needs to be updated to normalise the timezone to UTC+0 as per the DID specification and Timestamp needs more unit tests in general.

[cycraig]

This isn't really a breaking change is it? No APIs are changed since we didn't expose chrono from Timestamp unless I'm mistaken?

[olivereanderson]

This isn't really a breaking change is it? No APIs are changed since we didn't expose chrono from Timestamp unless I'm mistaken?

There are a few breaking changes introduced in this PR: One can no longer convert a chrono::ParseError to Error as a result of this change. The aforementioned change also replaces the wrapped value in the public enum which is another breaking change as far as I know. Furthermore the from_unix method is now fallible.

[cycraig]

Good points, I missed the from_unix change in particular.

[cycraig]

Thanks for the changes. Minor comments.