Index `Month` enum from 1

[pitdicker]

For 0.5 I'd like to make the Month enum less useless.

By defining Month::January as 1 users can do simple casting, and use it for example in:

let date = NaiveDate::from_ymd(2015, Month::March as u32, 14);

We can also choose to make Datelike::month return Month (#727). I didn't do so here for two reasons:

• For my personal projects using integers for months was perfectly fine and matches that year() and day() also return integers. I find working with integers more convenient than wordy month names.
• Returning en enum will introduce extra checks in the month method on NaiveDate that can't be optimized out without unsafe code. We know Mdf::from_ol(/* ... */).month() will reaturn a value in 1..=12, but the compiler doesn't.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.01%. Comparing base (08c9792) to head (bb658ae). Report is 5 commits behind head on 0.5.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.5.x #1551 +/- ## ========================================== + Coverage 93.98% 94.01% +0.03% ========================================== Files 37 37 Lines 16656 16579 -77 ========================================== - Hits 15654 15587 -67 + Misses 1002 992 -10 ```

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