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

Cargo.lock

@@ -76,9 +76,9 @@ checksum = "f8aac770f1885fd7e387acedd76065302551364496e46b3dd00860b2f8359b9d"
 
 [[package]]
 name = "backtrace"
-version = "0.3.40"
+version = "0.3.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "924c76597f0d9ca25d762c25a4d369d51267536465dc5064bdf0eb073ed477ea"
+checksum = "b4b1549d804b6c73f4817df2ba073709e96e426f12987127c48e6745568c350b"
 dependencies = [
  "backtrace-sys",
  "cfg-if",
@@ -1986,6 +1986,7 @@ name = "wasmtime"
 version = "0.9.0"
 dependencies = [
  "anyhow",
+ "backtrace",
  "cfg-if",
  "file-per-thread-logger",
  "libc",