Decouple some more Config methods from each other

[alexcrichton]

This commit decouples validation of stack sizes and guard sizes until
Engine::new to avoid odd interactions between the order of invocation
of Config methods.

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Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
  it.

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  Our documentation should be of the following form:

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  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?
  
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  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
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[cfallin]

LGTM!