Make methods on NaiveDateTime const where possible

[pitdicker]

I had to copy a few methods from the Datelike and Timelike traits to const methods on NaiveDate and NaiveTime, to make use of them in const methods on NaiveDateTime.

Otherwise the changes are fairly straightforward.

Only DateTime<Tz> to go after this (and a handful of methods that depend on Duration).

[codecov]

Codecov Report

Merging #1286 (6b3a314) into 0.4.x (c8e3eb4) will increase coverage by 0.01%.
The diff coverage is 96.49%.

@@            Coverage Diff             @@
##            0.4.x    #1286      +/-   ##
==========================================
+ Coverage   91.53%   91.55%   +0.01%     
==========================================
  Files          38       38              
  Lines       17325    17352      +27     
==========================================
+ Hits        15859    15886      +27     
  Misses       1466     1466              

Files	Coverage Δ
src/datetime/mod.rs	89.34% <100.00%> (+0.04%)	⬆️
src/naive/date.rs	96.25% <100.00%> (+0.03%)	⬆️
src/naive/time/mod.rs	96.55% <100.00%> (+0.02%)	⬆️
src/naive/datetime/mod.rs	97.64% <93.75%> (+<0.01%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[pitdicker]

The CI pointed out an issue because of our MSRV:

error[E0658]: trait bounds other than `Sized` on const fn parameters are unstable
   --> src\datetime\mod.rs:98:6
    |
98  | impl<Tz: TimeZone> DateTime<Tz> {
    |      ^^
...
123 |     pub const fn from_naive_utc_and_offset(datetime: NaiveDateTime, offset: Tz::Offset) -> DateTime<Tz> {
    |     --------------------------------------------------------------------------------------------------- function declared as const here
    |
    = note: see issue #93706 <https://github.com/rust-lang/rust/issues/93706> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `chrono` due to previous error

I have added a workaround so that we can have at least one constructor for a DateTime: NaiveDateTime::and_utc.

[jtmoon79]

Looks great.

[jtmoon79]

Just a fun recommendation: link to the meaning of "AD 1":

 /// ... with January 1, Year 1 (CE) as day 1, i.e. [AD 1](https://en.wikipedia.org/wiki/AD_1)

[djc]

Please squash the first three commits into the fourth one: they add code that goes unused.