wavm: emit stack trace when an exception happens

[mathetake]

this PR emits call stack trace when exception happens in WAVM just like V8 (#66)

this requires a change in the WAVM library used in Envoy: envoyproxy/envoy#13792

[2020-10-28 14:24:10.551][979699][trace][wasm] [source/extensions/common/wasm/wasm_vm.cc:29] Function proxy_on_tick failed:
WAVM message: wavm.reachedUnreachable
wasm backrace:
  1: wasm!helloworld!runtime._panic+7
  2: wasm!helloworld!command-line-arguments.C+2
  3: wasm!helloworld!command-line-arguments.B+0
  4: wasm!helloworld!command-line-arguments.A+0
  5: wasm!helloworld!(*command-line-arguments.helloWorld).OnTick+74
  6: wasm!helloworld!proxy_on_tick+0

Signed-off-by: mathetake takeshi@tetrate.io

[mathetake]

blocker: #49

We couldn't tell even if this compiles though I've already tested in my local machine. Before we merge this PR, I will write WAVM tests, and then test this PR

[PiotrSikora]

I don't think we need to block this on tests in this repo if you can open a PR in Envoy to make sure that this doesn't break anything.