time_t is now 64 bits. Strig(time_t) was ambiguous
tests added

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@mcspr I tried with charconv that I didn't know. That's very handy !
That one is faster (1.6x) but has a cost (+880B/flash+256B/rodata) (compared against lltoa)*
It has to be noted that these functions charconv(std::to_chars) or lltoa are used only when radix is not 10 (in which case sprintf is used). It should be quite rare.
Without further comments, I will push a newer commit keeping only lltoa. These functions will anyway not be linked in the general case.