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/aot/aot_runtime.c

@@ -1059,7 +1059,7 @@ aot_instantiate(AOTModule *module, bool is_sub_inst, uint32 stack_size,
 
 #if WASM_ENABLE_BULK_MEMORY != 0
 #if WASM_ENABLE_LIBC_WASI != 0
-    if (!module->is_wasi_module) {
+    if (!module->import_wasi_api) {
 #endif
         /* Only execute the memory init function for main instance because
             the data segments will be dropped once initialized.