Create wasmtime::Config from toml

[ludfjig]

This PR makes it possible to create a wasmtime::Config from a toml file and is a first step toward resolving #8784. Overall, I think the path forward is as follows:

1. This PR: Enables loading wasmtime::Config from a toml file, by making CommonOptions derive Deserialize. For example, opt_level is deserialized from values 0, 1, 2, "s" (same as cli flags) instead of "None", "Speed", "SpeedAndSize". Same applies to regalloc_algorithm, compiler and collector. Feedback appreciated on this.
2. Combine cache-config toml with this new toml.
3. Have one source of truth for cli flags and config options.

Feedback greatly appreciated, especially regarding how to deal with nn_graph, as well config_var and keyvalue_in_memory_data, which are all marked serde(skip) for now

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
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• If you added a new Config method, you wrote extensive documentation for
  it.

  Our documentation should be of the following form:

  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
  Is this method fallible? If so, when can it return an error?
  
  Can this method panic? If so, when does it panic?
  
  # Example
  
  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
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  For example, if you expose a new strategy for allocating the next instance
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  Often, all that is required of you is to ensure that there is a knob for this
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  of its nested structs).

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• If you are enabling a configuration option by default, make sure that it
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[alexcrichton]

Thanks for this! I'm liking how this is shaping up pretty cleanly so far. Would you be up for integrating this into a CLI flag itself as well? For example wasmtime --config cfg.toml or something like that

[alexcrichton]

This might perhaps be best as an inherent method on CommonOptions as an alternative constructor? That would still require the caller call .config(...) but that seems like a reasonable tradeoff to me.

[ludfjig]

I agree, moved to CommonOption::from_file instead. Happy to rename it to something different as well

[alexcrichton]

Mind updating the error using .with_context(...) to attach the name of the file here and below?

[ludfjig]

Good idea, addressed!

[ludfjig]

Thanks for this! I'm liking how this is shaping up pretty cleanly so far. Would you be up for integrating this into a CLI flag itself as well? For example wasmtime --config cfg.toml or something like that

Thanks for looking at this. I added a --config flag. Any suggestions on how to best test this?

[alexcrichton]

For tests I'd recommend tests/all/cli_tests.rs or somewhere around there for making a tempdir with a config file