Cap Sharing Improvements

[midnightveil]

Now that we have cap sharing with #406, some improvements that can be made:

• Experiment with removing the PD hierarchy feature entirely.
  • Specifically, we need a replacement for setting up the fault endpoints, but this
    does have a 1:1 mapping. (the cap_sharing is not good enough for this)
  • I would want the ability to automatically migrate users from hierarchy to cap sharing, i.e. run a program that converts system descriptor files.
• Make the "user caps" (MICROKIT_MAX_USER_CAPS=128) its own child cnode that is dynamically sized, there's no reason to limit this (see also: the untypeds PR).
  • This might end up with the microkit_cspace_slot_to_cptr function needing to be reworked...
  • Important: decide on this before the release, to avoid unnecessary future breakage.
• Auto-allocation of slots? (edit: maybe not?)
• Permissions on caps
• Other cap types

Originally posted by @midnightveil in #406 (comment)

[midnightveil]

Zoltan also happened to have agreed with removing the parent-child hierarchy as currently implemented in favour of some other way to ask for fault handler endpoints.