feat(wasix): Improve module loading performance and API correctness

[theduke]

This commit contains two notable changes:

• The VirtualFile trait is extended with a new as_owned_buffer() method.
  This allows cheap copying of file data if the file is already fully in
  memory, and enables optimisations for code that is aware of this
  functionality by allowing zero-copy cloning.
  This is especially useful for data that is already mmaped.

  The new method has an empty default impl, but is implemented for
  appropriate files (ReadOnlyFile, webc volume files).

• Modifies the Runtime::load_module method to take a new
  HashedModuleData struct

  This has multiple benefits:

  • HashedModuleData is safe by construction, thanks to private fields,
    so the hash it contains will always be valid for the module data.
    This removes a source of bugs due to passing a wrong hash for module
    data to module loaders etc.
  • HashedModuleData can be cheaply creating from a
    BinaryPackageCommand, preventing redundant cloning of data
  • The load_module method now receives owned data that is cheaply
    clonable.
    Previously it received a slice, which introduces the neccessity of
    cloning the data if it needs to be passed on to a different thread.

In combination this PR can significantly reduce memory usage for
high-concurrency scenarios.

Closes #5527

[promptless]

✅ No documentation updates required.