Replace boost with C++20

[alippai]

I have no experience with migrations like this. The tests pass, but it definitely needs a second pair of eyes.

[bgemmill]

Generally looks good to me, I had a few nits around nanosecond handling and epoch encocing, and it looks like all of this may just require cpp17 rather than 20.

Thanks for looping me in from the issue!

[alippai]

@bgemmill thanks for the help. I did my best to address the comments.

[xhochy]

Thanks! This looks really promising. Could you try and do some kind of performance test? The last time I removed some boost code the performance got worse and we needed to introduce simdutf as an additional dependency.