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_loader.h

@@ -24,8 +24,11 @@ extern "C" {
  * @return return module loaded, NULL if failed
  */
 WASMModule *
-wasm_loader_load(const uint8 *buf, uint32 size, char *error_buf,
-                 uint32 error_buf_size);
+wasm_loader_load(const uint8 *buf, uint32 size,
+#if WASM_ENABLE_MULTI_MODULE != 0
+                 bool main_module,
+#endif
+                 char *error_buf, uint32 error_buf_size);
 
 /**
  * Load a WASM module from a specified WASM section list.