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

samples/multi-module/wasm-apps/mA.c

@@ -1,5 +1,13 @@
-int
-A()
+__attribute__((export_name("A1"))) int
+A1()
 {
-    return 10;
-}
+    return 11;
+}
+
+int
+A2()
+{
+    return 12;
+}
+
+/* mA is a  reactor. it doesn't need a main() */