Refactor filesystem and block device system

[arthurp]

Currently, the filesystem stack as the following basic design. This is ignoring various details.

+--------------------+
| Posix Layer        |
+--------------------+
         |
         |
         v
+-------------------------------+                   +---------------------+
| File-backed Virtual Memory    |--- trait Pager -->| Page Cache Manager  |
+-------------------------------+                   +---------------------+
         |                                           |
         | (direct struct access)                    |
         v                                           |
+--------------------+                               |
| Page Cache         |<------------------------------+
+--------------------+
         |
         | trait PageStore
         v
+--------------------+
| File/"Inode"       |
+--------------------+
         |
         | (internal to specific filesystem)
         v
+--------------------+
| Filesystem         |
+--------------------+
         |
         | trait BlockDevice
         v
+--------------------+
| Block Device       |
+--------------------+

(Thanks ChatGPT for the cleaned up ASCII art.)

This should be changes to:

• Identical (or at least similar) interfaces between layers
• The cache as a separate server without multiple communication paths to other parts of the system.
• The cache can (and by default is) placed below the file system.
• The files are internal to the Filesystem server and only file handles are passed around outside that. The handle will be passed to the server to perform reads.

[arthurp]

I may have duplicated #120. I'm bad at issues.