benchfn dependencies reduced to only timefn

benchfn dependencies reduced to only timefn

benchfn used to rely on mem.h, and util,
which in turn relied on platform.h.
Using benchfn outside of zstd required to bring all these dependencies.

Now, dependency is reduced to timefn only.
This required to create a separate timefn from util,
and rewrite benchfn and timefn to no longer need mem.h.

Separating timefn from util has a wide effect accross the code base,
as usage of time functions is widespread.
A lot of build scripts had to be updated to also include timefn.