Debugging WebAssembly

The following steps describe a common way to debug a WebAssembly module in Wasmtime:

Compile your WebAssembly with debug info enabled, usually -g; for example:
```
clang foo.c -g -o foo.wasm
```
Run Wasmtime with the debug info enabled; this is -g from the CLI and Config::debug_info(true) in an embedding (e.g. see debugging in a Rust embedding)

Use a supported debugger:

lldb -- wasmtime run -g foo.wasm

gdb --args wasmtime run -g foo.wasm

If you run into trouble, the following discussions might help:

On MacOS with LLDB you may need to run: settings set plugin.jit-loader.gdb.enable on (#1953)
With LLDB, call __vmctx.set() to set the current context before calling any dereference operators (#1482):
```
(lldb) p __vmctx->set()
(lldb) p *foo
```
The address of the start of instance memory can be found in __vmctx->memory