Remove unused argument in wasm_runtime_lookup_function and refactor WASMModuleInstance (#3218)

Remove the unused parameter `signature` from `wasm_runtime_lookup_function`.

Refactor the layout of WASMModuleInstance structure:
- move common data members `c_api_func_imports` and `cur_exec_env` from
  `WASMModuleInstanceExtraCommon` to `WASMModuleInstance`
- In `WASMModuleInstance`, enlarge `reserved[3]` to `reserved[5]` in case that
  we need to add more fields in the future

ps.
https://github.com/bytecodealliance/wasm-micro-runtime/issues/2530
https://github.com/bytecodealliance/wasm-micro-runtime/issues/3202

doc/embed_wamr.md

@@ -100,7 +100,7 @@ After a module is instantiated, the runtime embedder can lookup the target WASM
 ```c
   /* lookup a WASM function by its name
      The function signature can NULL here */
-  func = wasm_runtime_lookup_function(module_inst, "fib", NULL);
+  func = wasm_runtime_lookup_function(module_inst, "fib");
 
   /* creat an execution environment to execute the WASM functions */
   exec_env = wasm_runtime_create_exec_env(module_inst, stack_size);

doc/multi_module.md

@@ -62,8 +62,7 @@ WAMR hopes that the native host or embedding environment loads/unloads the modul
 ```c
 wasm_function_inst_t
 wasm_runtime_lookup_function(wasm_module_inst_t const module_inst,
-                             const char *name,
-                             const char *signature);
+                             const char *name);
 ```
 
 Multi-module allows one to look up an exported function of a submodule. There are two ways to indicate the function _name_:

doc/perf_tune.md

@@ -288,7 +288,7 @@ And in the host embedder:
     bool ret;
 
     argv[0] = *(uint32 *)&arg_f32;
-    func = wasm_runtime_lookup_function(module_inst, "foo1", NULL);
+    func = wasm_runtime_lookup_function(module_inst, "foo1");
     ret = wasm_runtime_call_wasm(exec_env, func, 1, argv);
     if (!ret) {
         /* handle exception */