Get 'cash' working again

Now implemented via a hidden user called '-cash'.

This also introduces the concept of hidden accounts, that begin with '+' or
'-', for result accounts and balance accounts. Future versions can further
use this for more detailed bookkeeping. The idea behind the sign is that
'-' accounts should be inverted to get the intuitive value. So if the account
'-cash' has -13.37, that means there should be +13.37 in the cash box (or,
well, once the rest of this is implemented and the initial values are then set
correctly.)

lib/RevBank/Cart.pm

@@ -8,6 +8,7 @@ no warnings qw(experimental::signatures);
 use Carp ();
 use List::Util ();
 use RevBank::Global;
+use RevBank::Users;
 use RevBank::Cart::Entry;
 
 sub new($class) {
@@ -65,6 +66,12 @@ sub checkout($self, $user) {
         return;
     }
 
+    if ($user =~ /^[-+]/) {
+        # Hidden internal accounts
+        my $canonical = RevBank::Users::parse_user($user);
+        $user = $canonical // RevBank::Users::create($user);
+    }
+
     my $entries = $self->{entries};
 
     my %deltas;