--no-net -> --offline

Remove newline in operator table.

Reference #356.

Include note about type of catched errors in tryEval documentation

Add the caching mechanism already present for cli arguments to the
evaluation loop.

This means that an expression of the form `(builtins.getFlake
foo).bar.baz` will be cached if it can be

This is a dirty hack to make caching temporarily work.

The issue seems to be that the cache object is never deleted, which in
turn means that we never commit to the db (as it happens in the object's
destructor), so the cache doesn't persist between two Nix calls.
So we manually commit to the db each time. This is both slower and ugly,
but at least it works for the moment

The others don't need it and it was taking some useless space in the
`Value` type

Avoids sqlite lock issue when the same flake is evaluated twice by
keeping an in-memory mapping of the db connections opened for each cache
key

Re-order the initialisers in a constructor to make them match the
actual initialisation order