Sqlite

Adds:

• sqlite support (with a bunch of syscalls), and a test for it
• the /cwd directory is created on boot, which is automatically used whenever a pathname starting with a "." is encountered.
• the slab allocators are all disabled in favor of the linked list allocator for now. we should probably do a much more thoughtful reworking of the heap allocator.

The sqlite build happens automatically when you run cargo b, but is cached after. This means it now takes a minute or so to clean build pika.