Preserve full native stack traces in errors (#823)

* Preserve full native stack traces in errors

This commit builds on #759 by performing a few refactorings:

* The `backtrace` crate is updated to 0.3.42 which incorporates the
  Windows-specific stack-walking code, so that's no longer needed.
* A full `backtrace::Backtrace` type is held in a trap at all times.
* The trap structures in the `wasmtime-*` internal crates were
  refactored a bit to preserve more information and deal with raw
  values rather than converting between various types and strings.
* The `wasmtime::Trap` type has been updated with these various changes.

Eventually I think we'll want to likely render full stack traces (and/or
partial wasm ones) into error messages, but for now that's left as-is
and we can always improve it later. I suspect the most relevant thing we
need to do is to implement function name symbolication for wasm
functions first, and then afterwards we can incorporate native function
names!

* Fix some test suite assertions

crates/api/src/callable.rs

@@ -156,12 +156,7 @@ impl WrappedCallable for WasmtimeFn {
                 )
             })
         } {
-            let message = error.0;
-            let backtrace = error.1;
-
-            let trap = take_api_trap().unwrap_or_else(|| {
-                Trap::new_with_trace(format!("call error: {}", message), backtrace)
-            });
+            let trap = take_api_trap().unwrap_or_else(|| Trap::from_jit(error));
             return Err(trap);
         }