Re-export `EngineWeak` from the `wasmi` crate

[Robbepop]

Also properly mirror Wasmtime API for Engine::weak and EngineWeak docs.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.77%. Comparing base (d5f3a66) to head (03a2529).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1003 +/- ## ======================================= Coverage 81.77% 81.77% ======================================= Files 262 262 Lines 24378 24378 ======================================= Hits 19936 19936 Misses 4442 4442 ```

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