MIR: Add rollup base

Context

Just the very base of the rollup mostly following the tutorial, so that we write code with wasm target in mind.

See the commit description for details. Note that this MR depends on another one.

One unpleasant thing is that we'll need clang installed on our workstations, but that seems to be a fundamental restriction, Rust SDK tells about it too.

Further MRs:

• Add some initial code for handling messages;
• Elaborate (or create scripts) for running the kernel in some testable way.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR