zstd: Import upstream zstd-1.3.6

- Import zstd-1.3.6 from upstream
- Add zstd's module.c file
- Add the zstd module to Makefile.core.def

Import zstd-1.3.6 from upstream [1]. Only the files need for decompression
are imported. I used the latest zstd release, which includes patches [2] to
build cleanly in GRUB.

I included the script used to import zstd-1.3.6 below at the bottom of the
commit message.

Upstream zstd commit hash: 4fa456d7f12f8b27bd3b2f5dfd4f46898cb31c24
Upstream zstd commit name: Merge pull request #1354 from facebook/dev

Zstd requires some posix headers, which it gets from posix_wrap.
This can be checked by inspecting the .Po files generated by automake,
which contain the header dependencies. After building run the command
`cat grub-core/lib/zstd/.deps-core/*.Po` to see the dependencies [3].
The only OS dependencies are:

- stddef.h, which is already a dependency in posix_wrap, and used for size_t
  by lzo and xz.
- stdarg.h, which comes from the grub/misc.h header, and we don't use in zstd.

All the types like uint64_t are typedefed to grub_uint64_t under the hood.
The only exception is size_t, which comes from stddef.h. This is already the
case for lzo and xz. I don't think there are any cross-compilation concerns,
because cross-compilers provide their own system headers (and it would already
be broken).

[1] https://github.com/facebook/zstd/releases/tag/v1.3.6
[2] https://github.com/facebook/zstd/pull/1344
[3] https://gist.github.com/terrelln/7a16b92f5a1b3aecf980f944b4a966c4

```

curl -L -O https://github.com/facebook/zstd/releases/download/v1.3.6/zstd-1.3.6.tar.gz
curl -L -O https://github.com/facebook/zstd/releases/download/v1.3.6/zstd-1.3.6.tar.gz.sha256
sha256sum --check zstd-1.3.6.tar.gz.sha256
tar xzf zstd-1.3.6.tar.gz

SRC_LIB="zstd-1.3.6/lib"
DST_LIB="grub-core/lib/zstd"
rm -rf $DST_LIB
mkdir -p $DST_LIB
cp $SRC_LIB/zstd.h $DST_LIB/
cp $SRC_LIB/common/*.[hc] $DST_LIB/
cp $SRC_LIB/decompress/*.[hc] $DST_LIB/
rm $DST_LIB/{pool.[hc],threading.[hc]}
rm -rf zstd-1.3.6*
echo SUCCESS!
```

Signed-off-by: Nick Terrell <terrelln@fb.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/zstd/debug.c

@@ -0,0 +1,44 @@
+/* ******************************************************************
+   debug
+   Part of FSE library
+   Copyright (C) 2013-present, Yann Collet.
+
+   BSD 2-Clause License (http://www.opensource.org/licenses/bsd-license.php)
+
+   Redistribution and use in source and binary forms, with or without
+   modification, are permitted provided that the following conditions are
+   met:
+
+       * Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+       * Redistributions in binary form must reproduce the above
+   copyright notice, this list of conditions and the following disclaimer
+   in the documentation and/or other materials provided with the
+   distribution.
+
+   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+   You can contact the author at :
+   - Source repository : https://github.com/Cyan4973/FiniteStateEntropy
+****************************************************************** */
+
+
+/*
+ * This module only hosts one global variable
+ * which can be used to dynamically influence the verbosity of traces,
+ * such as DEBUGLOG and RAWLOG
+ */
+
+#include "debug.h"
+
+int g_debuglevel = DEBUGLEVEL;