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

tests/custom_signal_handler.rs

@@ -128,9 +128,12 @@ mod tests {
         {
             println!("calling read_out_of_bounds...");
             let trap = invoke_export(&instance, "read_out_of_bounds").unwrap_err();
-            assert!(trap
-                .message()
-                .starts_with("call error: wasm trap: out of bounds memory access"));
+            assert!(
+                trap.message()
+                    .starts_with("wasm trap: out of bounds memory access"),
+                "bad trap message: {:?}",
+                trap.message()
+            );
         }
 
         // these invoke wasmtime_call_trampoline from callable.rs
@@ -151,7 +154,7 @@ mod tests {
             let trap = read_out_of_bounds_func.call(&[]).unwrap_err();
             assert!(trap
                 .message()
-                .starts_with("call error: wasm trap: out of bounds memory access"));
+                .starts_with("wasm trap: out of bounds memory access"));
         }
         Ok(())
     }