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it has been replaced by DEBUGLEVEL,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.

lib/common/zstd_common.c

@@ -46,11 +46,6 @@ ZSTD_ErrorCode ZSTD_getErrorCode(size_t code) { return ERR_getErrorCode(code); }
  *  provides error code string from enum */
 const char* ZSTD_getErrorString(ZSTD_ErrorCode code) { return ERR_getErrorString(code); }
 
-/*! g_debuglog_enable :
- *  turn on/off debug traces (global switch) */
-#if defined(ZSTD_DEBUG) && (ZSTD_DEBUG >= 2)
-int g_debuglevel = ZSTD_DEBUG;
-#endif
 
 
 /*=**************************************************************