[trace] Keep track of a uint64_t tracing context

The most common information that you want to track between begin() and
end() is the timestamp of the begin function, so you can measure the
duration of the (de)compression call. Allow the tracing library to put
this information inside the `ZSTD_TraceCtx`, so it doesn't need to keep
a global map in this case. If a single uint64_t is not enough, the
tracing library can return a unique identifier (like the context
pointer) instead, and use it as a key in a map.

This keeps the simple case simple.

lib/decompress/zstd_decompress_internal.h

@@ -21,6 +21,7 @@
  *********************************************************/
 #include "../common/mem.h"             /* BYTE, U16, U32 */
 #include "../common/zstd_internal.h"   /* ZSTD_seqSymbol */
+#include "../common/zstd_trace.h"      /* ZSTD_TraceCtx */
 
 
 
@@ -178,7 +179,9 @@ struct ZSTD_DCtx_s
 #endif
 
     /* Tracing */
-    int tracingEnabled;
+#if ZSTD_TRACE
+    ZSTD_TraceCtx traceCtx;
+#endif
 };  /* typedef'd to ZSTD_DCtx within "zstd.h" */