Expose the WebAssembly runtime to applets

[ia0]

Applets may want to run sandboxed code. It would be convenient for them to reuse the WebAssembly runtime of the scheduler. This could be useful for example to execute a policy.

Design questions:

• The applet would need to provide host functions in some form.
• Should the new module be executed in the same store as the applet? Probably not.
• What is the performance cost of starting a new store?
• Should we run in a different execution context? (related to #78)