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/instance.rs

@@ -52,8 +52,8 @@ fn instantiate_in_context(
     let instance = compiled_module.instantiate().map_err(|e| -> Error {
         if let Some(trap) = take_api_trap() {
             trap.into()
-        } else if let InstantiationError::StartTrap(msg) = e {
-            Trap::new(msg).into()
+        } else if let InstantiationError::StartTrap(trap) = e {
+            Trap::from_jit(trap).into()
         } else {
             e.into()
         }