Add ZSTD_minCLevel, like ZSTD_maxCLevel

[cemeyer]

Minor enhancement request.

Especially now that ZSTD API has negative ("fast") compression levels, I think it makes sense to give callers a lower bound too.

Mentioned in #89 as a "when this changes in the future."

[Cyan4973]

We would need to define a minimum compression level first !
But indeed, I believe that's a reasonable request.

[cemeyer]

INT_MIN ? 😂 😂

Thanks :-).

[Cyan4973]

function ZSTD_minCLevel() is now defined in dev branch.