Fix Wasm from rust docs

[austinabell]

Hello!

I was playing around with this and noticed the docs are a bit broken. Here are my changes:

• Update wasmtime version in docs to 0.16.0
  • Breaking API changes from Refactor #1524 were made in the docs, but the old version was kept
• Change second code wat example.
  • The run function does not return anything (I guess another option would be to return some value to keep the function call the same signature as previous but I think this may help some)
• Expose the whole file code for the updated usage on the second example
  • Expanding from the previous example, it is confusing how the code is setup for this, as there is the other changes needed for this to run successfully
  • Open to suggestions or reverting this, as this is more opinionated

Link to docs for reference, if it helps: https://bytecodealliance.github.io/wasmtime/lang-rust.html

[alexcrichton]

Thanks!