grouped debug functions into debug.h

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it has been replaced by DEBUGLEVEL,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.

tests/fuzz/fuzz.h

@@ -23,10 +23,10 @@
  *        the data to zstd functions. Every fuzzer initializes the RNG exactly
  *        once before doing anything else, even if it is unused.
  *        Default: 4.
- * @param ZSTD_DEBUG:
- *        This is a parameter for the zstd library. Defining `ZSTD_DEBUG=1`
+ * @param DEBUGLEVEL:
+ *        This is a parameter for the zstd library. Defining `DEBUGLEVEL=1`
  *        enables assert() statements in the zstd library. Higher levels enable
- *        logging, so aren't recommended. Defining `ZSTD_DEBUG=1` is
+ *        logging, so aren't recommended. Defining `DEBUGLEVEL=1` is
  *        recommended.
  * @param MEM_FORCE_MEMORY_ACCESS:
  *        This flag controls how the zstd library accesses unaligned memory.

tests/fuzz/fuzz.py

@@ -251,7 +251,7 @@ def build_parser(args):
         dest='debug',
         type=int,
         default=1,
-        help='Set ZSTD_DEBUG and BIT_DEBUG (default: 1)')
+        help='Set DEBUGLEVEL (default: 1)')
     parser.add_argument(
         '--force-memory-access',
         dest='memory_access',
@@ -358,8 +358,7 @@ def build(args):
     common_flags = []
 
     cppflags += [
-        '-DZSTD_DEBUG={}'.format(args.debug),
-        '-DBIT_DEBUG={}'.format(args.debug),
+        '-DDEBUGLEVEL={}'.format(args.debug),
         '-DMEM_FORCE_MEMORY_ACCESS={}'.format(args.memory_access),
         '-DFUZZ_RNG_SEED_SIZE={}'.format(args.fuzz_rng_seed_size),
     ]