Improve return value types in the C API

[gzurl]

@ereslibre initiated an interesting discussion about the return value types in the C API.

Currently, some functions return a *const c_char as a result:

pub extern "C" fn wasm_executionctx_run(executionctx_id: *const c_char) -> *const c_char
pub extern "C" fn wasm_executionctx_create_from_config(config_id: *const c_char) -> *const c_char {

So there is no formal way to distinguish whether the returned data is the expected output or a text explaining the error.

We should explore returning a CResult that looks like this:

struct CResult {
  bool succeeded;
  char *result;
};

Or we could go for this other approach of result-type-in.c.

An example of mod_wasm.c code that is missing error logging is the Wasm execution invocation. We don't know if the returned string was the expected output or not.

const char* module_response = wasm_executionctx_run(exec_ctx_id);

In any case, we should take care of returning the string ownership to Rust.

I am opening this issue to have a discussion about it.