Improve trap error messages (#831)

* Improve trap error messages

The new trap error message for the issue #828 looks like:

```
thread 'main' panicked at 'a', /proc/self/fd/11:1:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
Error: failed to run main module `test.wasm`

Caused by:
    0: failed to invoke `_start`
    1: wasm trap: unreachable, source location: @6cea
       wasm backtrace:
         0: __rust_start_panic
         1: rust_panic
         2: std::panicking::rust_panic_with_hook::h57f0cff11449798f
         3: std::panicking::begin_panic::hd620695467c5dd1f
         4: test::main::ha54db001eabbde1b
         5: std::rt::lang_start::{{closure}}::h5acfb82693695869
         6: std::sys_common::backtrace::__rust_begin_short_backtrace::h39e8b9420da241f9
         7: std::panicking::try::do_call::hb7ebfcd70d5f703e
         8: __rust_maybe_catch_panic
         9: std::rt::lang_start_internal::hd5f64f52a5c5315c
         10: std::rt::lang_start::h2a51d79994dd0c4b
         11: __original_main
         12: _start
```

Closes #828

* Tidy up the style of the traps tests

* Add some tests and module names

crates/api/Cargo.toml

@@ -19,6 +19,7 @@ region = "2.0.0"
 libc = "0.2"
 cfg-if = "0.1.9"
 backtrace = "0.3.42"
+rustc-demangle = "0.1.16"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"