This isn't finished but I'll commit as I go. We need to get to where context
switching only needs to change CR3 and everything's ready to go.
My basic idea is:
- The first 4 kB is off-limits. This catches null dereferences.
- Up to the 4 MB mark is identity-mapped and kernel-only.
- The rest is available to everyone!
While the first 4 MB is only available to the kernel, it's still mapped in
every process, for convenience when entering the kernel.
I also added a generator cache to FileHandle. This way, multiple
reads to a generated file (i.e in a synthfs) can transparently
handle multiple calls to read() without the contents changing
between calls.
The cache is discarded at EOF (or when the FileHandle is destroyed.)
I know I'm praying for cargo here, but this does fix a weird issue
where logging the sum_alloc and sum_free globals wouldn't display
symmetric values all the time.
