created ZSTDMT_sizeof_CCtx() and POOL_sizeof()

required by ZSTD_sizeofCCtx() while adding a ZSTDMT_CCtx*

lib/common/pool.c

@@ -146,6 +146,13 @@ void POOL_free(POOL_ctx *ctx) {
     free(ctx);
 }
 
+size_t POOL_sizeof(POOL_ctx *ctx) {
+    if (ctx==NULL) return 0;  /* supports sizeof NULL */
+    return sizeof(*ctx)
+        + ctx->queueSize * sizeof(POOL_job)
+        + ctx->numThreads * sizeof(pthread_t);
+}
+
 void POOL_add(void *ctxVoid, POOL_function function, void *opaque) {
     POOL_ctx *ctx = (POOL_ctx *)ctxVoid;
     if (!ctx) { return; }

lib/common/pool.h

@@ -32,6 +32,11 @@ POOL_ctx *POOL_create(size_t numThreads, size_t queueSize);
 */
 void POOL_free(POOL_ctx *ctx);
 
+/*! POOL_sizeof() :
+    return memory usage of pool returned by POOL_create().
+*/
+size_t POOL_sizeof(POOL_ctx *ctx);
+
 /*! POOL_function :
     The function type that can be added to a thread pool.
 */