Wasmtime: support a notion of "custom code publisher".

[cfallin]

In some no_std environments, virtual memory usage is generally prohibited for performance-predictability reasons, but the MMU hardware is still in use for permissions (e.g., W^X write-xor-execute). Occasional changes to page mapping permissions are thus necessary when new modules are loaded dynamically, and are acceptable in that context. Wasmtime needs a way to support "publishing" code (making it executable) in such environments.

Rather than try to segment the signals-based-traps divide further, and piece out the code-publishing parts from the heap parts, and backdoor a path to mprotect in an otherwise no_std build, in this PR I have opted to add a trait an impl of which the embedder can provide to the Config to implement custom actions for "code publish". This otherwise operates properly in a no-signals-based-traps environment, e.g., the module backing memory itself is regularly allocated rather than mmap'd (but is now aligned to the degree requested by the trait impl).

[cfallin]

Updated, thanks!

[alexcrichton]

Would it be possible to add a test exercising this trait as well? Perhaps x86_64-linux-specific to avoid rewriting a bunch of logic to make things executable or something like that.

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[cfallin]

Updated, thanks for feedback! Added a test that uses rustix on non-Windows targets to delegate to mprotect ("custom but actually the same as always" implementation).