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/Cargo.toml

@@ -18,6 +18,7 @@ anyhow = "1.0.19"
 region = "2.0.0"
 libc = "0.2"
 cfg-if = "0.1.9"
+backtrace = "0.3.42"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"