rdump: First command 'create'

Accept a path, and construct a pool at that path.
sql: Create `sql_one` utility

This utility performs an sql query that expects a single result row.
Returns an option for either present, or not.
pool: Basic FilePool opening

Opens the file pool, and reads the UUID from it.
Primitive 'list' command

Prints out the hashes of the present backups in the given pool.
Implement simple chunk sourcing

Read chunks that are inline in the database.  The 'list' command can
print these.
pool: Implement `Container`

Probably not very useful, but it makes it consistent.