Enable env_logger in the C API

[alexcrichton]

This commit ensures that env_logger and RUST_LOG are configured to
work with the C API.

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[abrown]

I haven't been drop-ing this type of call; should I be?

[alexcrichton]

Oh heh good point! I've added a comment.

The drop here isn't necessary in all cases, it's just our case where try_init fails after the first call, and it's expected that you should be able to call wasm_engine_new multiple times.