Add WASI ABI compatibility check for multi-module (#913)

Refer to https://github.com/WebAssembly/WASI/blob/main/design/application-abi.md
to check the WASI ABI compatibility:
- Command (main module) may export _start function with signature "()"
- Reactor (sub module) may export _initialize function with signature "()"
- _start and _initialize can not be exported at the same time
- Reactor cannot export _start function
- Command and Reactor must export memory

And
- Rename module->is_wasi_module to module->import_wasi_api
- Refactor wasm_loader_find_export()
- Remove MULTI_MODULE related codes from mini_loader
- Update multi-module samples
- Fix a "use-after-free" issue. Since we reuse the memory instance of sub module,
   just to protect it from freeing an imported memory instance

core/iwasm/interpreter/wasm_runtime.h

@@ -40,11 +40,6 @@ struct WASMMemoryInstance {
     /* The heap created */
     void *heap_handle;
 
-#if WASM_ENABLE_MULTI_MODULE != 0
-    /* to indicate which module instance create it */
-    WASMModuleInstance *owner;
-#endif
-
 #if WASM_ENABLE_SHARED_MEMORY != 0
     /* mutex lock for the memory, used in atomic operation */
     korp_mutex mem_lock;