Call process::exit in WASI proc_exit syscall.

[kanaka]

Currently calling proc_exit causes an exception and does not set the exit code. This updates the syscall to use std::process::exit instead of calling Err.

Fixes: #361

[lachlansneff]

Wasmer is designed to run multiple tenants and as a result, the WASI integration is also designed to be multi-tenant. Calling the exit syscall will just kill the process, even if there are other instances running.

There's some functionality missing in wasmer that needs to be implemented that will allow the runtime to retrieve the exit code from the thrown exception, which will fix this issue.

[kanaka]

@lachlansneff ah, so I assume that the if there is only a single tenant then when that tenant exits, it's exit code (if there is one) becomes exit code of the wasmer process. If you have multiple tenants, I assume you would delay exit until all tenants have exited? Out of curiosity do you just pick the exit code of the last tenant "thread" to be the wasmer exit code?

I'll close this PR and just wait (impatiently) for the original issue to be addressed the right way for wasmer's architecture :-)

[lachlansneff]

This issue will be fixed today, promise!

The wasmer cli program is one-tenant only right now, so it'll just return the exit code of that tenant.