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it has been replaced by DEBUGLEVEL,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.

Makefile

@@ -64,7 +64,7 @@ zlibwrapper:
 
 .PHONY: test
 test:
-	$(MAKE) -C $(PRGDIR) allVariants MOREFLAGS+="-g -DZSTD_DEBUG=1"
+	$(MAKE) -C $(PRGDIR) allVariants MOREFLAGS+="-g -DDEBUGLEVEL=1"
 	$(MAKE) -C $(TESTDIR) $@
 
 .PHONY: shortest