Rearrange workspace to allow import of mentat crate in cli crate
Create store object inside repl when started for connecting to mentat
Use provided DB name to open connection in store
Accept DB name as command line arg.
Open on CLI start
Implement '.open' command to open desired DB from inside CLI
This doesn't yet introduce a working Cargo.toml for 'mentatweb', but it
does allow RLS to build correctly without errors, and it reduces the
core library's dependency space, which is more important in the short
term.
