The regression tests run nightly or on the `regression`
branch for convenience. The results get uploaded as the
artifacts of the job. If they change, check the diff
printed in the job. If all is well, download the new
results and commit them to the repo.

This code will only run on a UNIX like platform. It
could be made to run on Windows, but I don't think that
it is necessary. It also uses C99.

* data: This module defines the data to run tests on.
  It downloads data from a URL into a cache directory,
  checks it against a checksum, and unpacks it. It also
  provides helpers for accessing the data.
* config: This module defines the configs to run tests
  with. A config is a set of API parameters and a set of
  CLI flags.
* result: This module is a helper for method that defines
  the result type.
* method: This module defines the compression methods
  to test. It is what runs the regression test using the
  data and the config. It reports the total compressed
  size, or an error/skip.
* test: This is the test binary that runs the tests for
  every (data, config, method) tuple, and prints the
  results to the output file and stderr.
* results.csv: The results that the current commit is
  expected to produce.

Fix i386 build failure "Junk character 13"

- Factor out LDM's hash function for reuse
- Add rsyncable mode to zstdmt and expose it via the advanced API
- Fix `-B`/`--block-size` to actually set the job size
- Add rsyncable tests to `zstreamtest` and `playTests.sh`

Tested by:

```
> cat A.100MB B.100MB C.100MB D.100MB E.100MB | zstd --rsyncable -fo src/file.zst
/*stdin*\            : 48.22%   (524288000 => 252837782 bytes, src/file.zst)
> rsync -rc --stats src devbigvm:/data/users/terrelln/rsync-test
Total bytes sent: 252868779
total size is 252837782  speedup is 1.00
> echo test > test
> cat test A.100MB test B.100MB test C.100MB test D.100MB test E.100MB | zstd --rsyncable -fo src/file.zst
/*stdin*\            : 48.23%   (524288025 => 252838025 bytes, src/unicorn.tar.zst)
> rsync -rc --stats src devbigvm:/data/users/terrelln/rsync-test
Total bytes sent: 4605696
total size is 252838025  speedup is 53.60
```

Close #1155.

- Add rsyncable mode to multithreaded mode
- Factor out LDM's hash function for reuse

Separating benchfn and benchzstd

because gcc considers that
`void* const *`  and `void**` are 2 different types ...

to use structure for function parameters
as it expresses much clearer than a long list of parameters,
since each parameter can now be named.

with BMK_extract_returnValue()

it shall be possible to use benchfn
without any dependency on zstd.

Fix #1412: Perform Signed Comparison When Setting Attach Dict Param

which is more accurate

Improves decompression speed when using cold dictionary

 improve long-range decoder speed

by triggering the prefetching decoder path
(which used to be dedicated to long-range offsets only).

Figures on my laptop :
no content prefetch : ~300 MB/s (for reference)
full content prefetch : ~325 MB/s (before this patch)
new prefetch path : ~375 MB/s (after this patch)

The benchmark speed is already significant,
but another side-effect is that this version
prefetch less data into memory,
since it only prefetches what's needed, instead of the full dictionary.

This is supposed to help highly active environments
such as active databases,
that can't be properly measured in benchmark environment (too clean).

Also :
fixed the largeNbDict test program
which was working improperly when setting nbBlocks > nbFiles.

Add TSAN dictionary tests

one limit case can apparently be generated during fuzzer tests

as it was broken and no one noticed

on enwik9 at level 22 (which is almost a worst case scenario),
speed improves by +7% on my laptop (415 -> 445 MB/s)

Prevent deadlock on malloc() failure.

Allow creating debug builds with CMake