Add a reference counter to wasm_engine_t (#1001)

This patch allows safer (note: safer, not safe) embedding in a plugin
environment where multiple instances of the engine could be needed.

Original code initializes and tears down the full runtime during
wasm_engine_new() and wasm_engine_delete() respectively. After this
update the C API implementation keeps track of engine instances count
and inits/deinits the runtime only when needed.

This allows for example to call wasm_engine_new() twice and then call
wasm_engine_delete() once without rendering the first engine instance
invalid.

core/iwasm/common/wasm_c_api.c

@@ -320,6 +320,7 @@ wasm_engine_new()
         singleton_engine =
             wasm_engine_new_internal(Alloc_With_System_Allocator, NULL);
     }
+    singleton_engine->ref_count++;
     return singleton_engine;
 }
 
@@ -336,6 +337,7 @@ wasm_engine_new_with_args(mem_alloc_type_t type, const MemAllocOption *opts)
     if (!singleton_engine) {
         singleton_engine = wasm_engine_new_internal(type, opts);
     }
+    singleton_engine->ref_count++;
     return singleton_engine;
 }
 
@@ -343,7 +345,7 @@ wasm_engine_new_with_args(mem_alloc_type_t type, const MemAllocOption *opts)
 void
 wasm_engine_delete(wasm_engine_t *engine)
 {
-    if (engine) {
+    if (engine && (--engine->ref_count == 0)) {
         wasm_engine_delete_internal(engine);
         singleton_engine = NULL;
     }

core/iwasm/common/wasm_c_api_internal.h

@@ -27,6 +27,7 @@ WASM_DECLARE_VEC(store, *)
 struct wasm_engine_t {
     /* support one store for now */
     wasm_store_vec_t *stores;
+    uint32_t ref_count;
 };
 
 struct wasm_store_t {