* Updates CircleCI to use workflows.
We can now specify any number of test jobs to run in parallel.
* Switch the image to `buildpack-deps:trusty` which is only 500 MB
instead of 7 GB, so that saves 7 minutes to download it if it isn't
already cached on the host.
* Publish the source tarball and sha256sum as artifacts.
* If the `GITHUB_TOKEN` environment variable is set, we will also
add the tarball + sha256sum to the tagged release, after manual
approval.
There were 2 competing set of debug functions
within zstd_internal.h and bitstream.h.
They were mostly duplicate, and required care to avoid messing with each other.
There is now a single implementation, shared by both.
Significant change :
The macro variable ZSTD_DEBUG does no longer exist,
it has been replaced by DEBUGLEVEL,
which required modifying several source files.
