Result alias for convienient use of anyhow::Error without depending on anyhow

[rylev]

As discussed on Zulip, the bindgen! macro assumes the usage of anyhow. This adds a type alias to wasmtime for std::result::Result<T, anyhow::Error> that allows users to use wasmtime::Result where a std::result::Result<T, anyhow::Error> is expected.

In particular, this means that users of bindgen! can use wasmtime::Result when they are implementing an import function.

One caveat: after making this change, I realized that the bindgen! macro has the trappable_error_type option which could be used to change the error type assume. While some changes need to be made no matter what (as many of the methods generated by the macro still assume anyhow and don't respect that option), we might want to consider not adding this type alias and just making it clearer that users should use the trappable_error_type option if they are not using anyhow.

r? @alexcrichton

[pchickey]

I'm fine with this, Alex what do you think?

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[rylev]

@alexcrichton I applied your suggestion!