Add support for initial dynamicism (PD hierarchy)

[Ivan-Velickovic]

This PR rebases an old branch (https://github.com/seL4/microkit/tree/pd-hierarchy) and gets it working with the current tool.

This allows for PDs to be 'parents' of other PDs and handle faults from child PDs.

There are also new libmicrokit APIs for controlling the execution of a child PD.

This PR is a prerequisite for virtualisation support.

Before merging:

• [x] Finish manual changes
• [x] Add XML parsing tests for child PDs
• [x] Remove TODOs in code that I've added