[tz] Direct implementation of mktime_z

[Ted Phelps]

Hi Tony,

On 2022-09-24, Tony Finch wrote:
> You might also like https://dotat.at/@/2008-09-10-counting-the-days.html
> (That does things the hard way, rather than relying on tm_yday)
> and https://dotat.at/@/2008-09-15-the-date-of-the-count.html

Wow, that's some clever math you've done there; far nicer than the 
clumsy approach I'd used to canonicalize the year, month and day.  It's 
a little slower than my clumsy approach in the case where the year, 
month and day are already in canonical form, but can be much faster when 
they're not.  I'll incorporate those changes into my git project in the 
coming days, thank you!

-Ted