Disk blocks

Disk blocks are Collapse OS' main access to permanent storage.
The system is exceedingly simple: blocks are contiguous 
chunks of 1024 bytes each living on some permanent media such
as floppy disks or SD cards. They are mostly used for text,
either informational or source code, which is organized into
16 lines of 64 characters each.

Blocks are referred to by number, 0-indexed. They are read
through BLK@ and written through BLK!. When a block is read,
its 1024 bytes content is copied to an in-memory buffer
starting at BLK( and ending at BLK). Those read/write
operations are often implicit. For example, LIST calls BLK@.

                                                        (cont.)
