Add From<NaiveDate> for NaiveDateTime

[dcechano]

This PR implements From<NaiveDate> to NaiveDateTime to facilitate more flexible APIs.

Motivation

Some users of the chrono library may want to create functions that can take both NaiveDate and NaiveDateTime. Without this trait that is not necessarily possible. Currently NaiveDate implements From<NaiveDateTime> so they could build an API that takes NaiveDateTimes and converts them NaiveDates but the reverse is not possible.

Currently only this is possible:

fn use_as_naive_date<D>(date: D) -> NaiveDate
where
    D: Into<NaiveDate>,
{
    date.into()
}

The function above can handle NaiveDate or NaiveDateTime input where the body can use the input to get a NaiveDate.
With my changes, this will be possible as well:

fn use_as_naive_datetime<D>(date: D) -> NaiveDateTime
where
    D: Into<NaiveDateTime>,
{
    date.into()
}

Here the use has created a function that can take NaiveDate or NaiveDateTime and get a NaiveDateTime. While ostensibly a small change, it gives chrono users the ability to create more powerful and flexible APIs.

Thank you for considering my pull request.

[codecov]

Codecov Report

Merging #1355 (cefa644) into 0.4.x (c0f418b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            0.4.x    #1355   +/-   ##
=======================================
  Coverage   91.61%   91.62%           
=======================================
  Files          38       38           
  Lines       17489    17497    +8     
=======================================
+ Hits        16023    16031    +8     
  Misses       1466     1466           

Files	Coverage Δ
src/naive/datetime/mod.rs	97.75% <100.00%> (+0.01%)	⬆️

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[djc]

This seems reasonable, thanks!

[pitdicker]

A late reply, but cool trick to use Into<NaiveDateTime> as a bound!