`TimeZone` trait: take `NaiveDateTime` by value

[pitdicker]

Adjust TimeZone::{offset_from_local_datetime, offset_from_utc_datetime, from_local_datetime, from_utc_datetime} to take their NaiveDateTime argument by value. Taking a small Copy value by reference was useless friction.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.36641% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 94.01%. Comparing base (d286f93) to head (d035716).

Files	Patch %	Lines
src/datetime/tests.rs	87.27%	7 Missing :warning:
src/datetime/mod.rs	78.57%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.5.x #1522 +/- ## ========================================== - Coverage 94.01% 94.01% -0.01% ========================================== Files 37 37 Lines 16830 16824 -6 ========================================== - Hits 15823 15817 -6 Misses 1007 1007 ```

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