Add Date::atStartOfDay function.

[mwz]

This PR adds a new Date function atStartOfDay, which turns an existing date into a new date with the time set to the start of the day, i.e. sets the time to midnight. Please see the example below:

The existing date functions don't seem to be sorted alphabetically, so I added the new function at the end, please let me know if you'd like me to move it somewhere else; the same applies to the test (is one test sufficient?).

This is my first piece of code in OCaml, so I'd be really happy to get any feedback.

Relates to #2411.

[mwz]

execute_function_works integration test seems to have failed, but the name indicates it's flaky, hopefully nothing related to this PR.

[pbiggar]

Awesome, this is great, thanks!