feat: add NAIVE_UNIX_EPOCH constant to PrimitiveDateTime

[xoac]

I am not sure if this is acceptable in terms of interface time crate want to provide. I think NAIVE prefix is nice suggestion that developer is responsible for correctness of logic here and such constant can be useful.

[codecov]

Codecov Report

Merging #569 (88cf46f) into main (4507d07) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #569   +/-   ##
=====================================
  Coverage   95.7%   95.7%           
=====================================
  Files         79      79           
  Lines       8768    8780   +12     
=====================================
+ Hits        8394    8406   +12     
  Misses       374     374           

Impacted Files	Coverage Δ
time/src/date_time.rs	94.6% <ø> (ø)
time/src/primitive_date_time.rs	100.0% <ø> (ø)
time/src/weekday.rs	100.0% <100.0%> (ø)

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[jhpratt]

"Unix epoch" fundamentally requires a time zone, as it measures time since 1970-01-01 0:00 UTC. If a user wishes to use the value without a time zone, it can be trivially constructed (at compile-time) using datetime!(1970-01-01 0:00).

In the future, I intend on implementing additional functionality such that it's possible to have a statically-known offset with zero runtime overhead. However, this likely requires language features that are not near stabilization.