Non-mutating Datetime fns take a pointer to self

[hazeycode]

Member fns of Datetime take pointer type as self but do no mutation. This forces all instances to be declared var and does not allow const Datetime's.

These fns should just take a value type, there's virtually no benefit to passing a pointer to self in these cases; since the size of Datetime is very small, and in many cases the compiler can inline the fns anyway.

Datetime.Delta.totalSeconds,
Dateime.formatHttp,
Datetime.formatHttpBuf

[frmdstryr]

Thanks for the report! Updated now. If you see anything else let me know.

[hazeycode]

Wow, that was quick @frmdstryr ! I was going to open a PR, but thanks!

[hazeycode]

super-minor: maybe the README example should be updated to inline with this change

[hazeycode]

Nice