[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/common/zstd_trace.c

@@ -15,27 +15,27 @@
 
 #if ZSTD_TRACE && ZSTD_HAVE_WEAK_SYMBOLS
 
-ZSTD_WEAK_ATTR int ZSTD_trace_compress_begin(ZSTD_CCtx const* cctx)
+ZSTD_WEAK_ATTR ZSTD_TraceCtx ZSTD_trace_compress_begin(ZSTD_CCtx const* cctx)
 {
     (void)cctx;
     return 0;
 }
 
-ZSTD_WEAK_ATTR void ZSTD_trace_compress_end(ZSTD_CCtx const* cctx, ZSTD_trace const* trace)
+ZSTD_WEAK_ATTR void ZSTD_trace_compress_end(ZSTD_TraceCtx ctx, ZSTD_Trace const* trace)
 {
-    (void)cctx;
+    (void)ctx;
     (void)trace;
 }
 
-ZSTD_WEAK_ATTR int ZSTD_trace_decompress_begin(ZSTD_DCtx const* dctx)
+ZSTD_WEAK_ATTR ZSTD_TraceCtx ZSTD_trace_decompress_begin(ZSTD_DCtx const* dctx)
 {
     (void)dctx;
     return 0;
 }
 
-ZSTD_WEAK_ATTR void ZSTD_trace_decompress_end(ZSTD_DCtx const* dctx, ZSTD_trace const* trace)
+ZSTD_WEAK_ATTR void ZSTD_trace_decompress_end(ZSTD_TraceCtx ctx, ZSTD_Trace const* trace)
 {
-    (void)dctx;
+    (void)ctx;
     (void)trace;
 }