Implement start_of_* and end_of_* APIs

[liquidscorpio]

Context

Back in 2015, a similar request for a date transition API was closed. I wish to re-open conversation for a more generalized API for the same. Such APIs are utilized extensively in time-series & analytical workloads and with chrono almost being the canonical date-time crate for Rust, there is value to be had by having such APIs within the library.

I am already building this and would like put forth a PR for the same. In the meanwhile, checking if the stance of the team on the issue has changed?

Public Interface

Support for following methods on a NaiveDate instance for the following APIs:

start_of_year() // Transforms 2019-07-12 to 2019-01-01
end_of_year() // Transforms 2019-07-12 to 2019-31-12
start_of_month()  // Transforms 2019-07-12 to 2019-07-01
end_of_month()  // Transforms 2019-07-12 to 2019-07-31
start_of_week()   // Transforms 2019-07-12 to 2019-07-08
end_of_week() // Transforms 2019-07-12 to 2019-07-14

All the APIs can also have start_of_pred_* / end_of_pred_* / start_of_succ_* / end_of_succ_* keeping things consistent.

While I am explicitly referring to NaiveDate, other structs can also be considered one at a time.

[quodlibetor]

Yes, the stance has changed, I would love a PR for this!

[liquidscorpio]

Work relating to this have been moved to a new crate.
See: #385 (comment) for more information.