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.
some non-trivial changes to platform.h and util.h,
initially related to compilation for Haiku,
but I used this opportunity to make them cleaner
and add some documentation.
Noticed several tests that could be improved
(too harsh conditions, useless exception, etc.)
but I did not dare modifying too many tests just before release.
by invoking time() once per batch, instead of once per compression / decompression.
Batch is dynamically resized so that each round lasts approximately 1 second.
Also : increases time accuracy to nanosecond
The new macro might be a bit too restrictive.
Systems which do not support new test will simply default to <time.h>'s `clock_t clock()`,
suffering lesser benchmark accuracy.
Should it matter, the detection macro will have to be upgraded.
Currently, all files are joined by default,
they are compressed separately but benchmarked together,
providing a single final result.
Benchmarking files separately make it possible to accurately measure difference for each file.
This is expected to be useful while tuning optimal parser.
UTIL_getFileSize() used to return zero on failure.
This made it impossible to distinguish a failure from a genuine empty file.
Both cases where coalesced.
Adding UTIL_FILESIZE_UNKNOWN constant has many consequences on user code,
since in many places, the `0` was assumed to mean "error".
This is no longer the case, and the error code must be actively checked.
for easier invocation.
- no longer expose frequency timer :
it's either useless, or stored internally in a static variable (init is only necessary once).
- UTIL_getTime() provides result by function return.
The timer used was only accurate up to 0.01 seconds. This timer is accurate up to 1 ns.
It is a monotonic timer that measures the real time difference, not on CPU time.
