Libkernel yet more fixes

[hexagonal-sun]

libkernel: region: add new utility functions

Add the following to all memory region variants:

 - `is_empty()`: Return true if the region has zero size.
 - `cap_size()`: Reduces the size if the given size < current.
 - `shrink_start()`: Move start address forward, while shrinking the
   region.

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libkernel: smalloc: don't ever allocate a null address

If a memory list begins at NULL, never give out that address. It trips
up UB checks, instead reserve the null page when that condition has been
detected.

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libkernel: paging: add `translate`

Add a `translate` function which is much more powerful that `get_pte`.
It permits block mappings to be returned for a given VA.