Add type declaration for singleton field

[trapd00r]

Problem

Since singleton is a binary flag similar to comp, it should return results the same way comp does. As of now, they do differ:

% beet ls comp:1|wc -l
15163

% beet ls comp:true|wc -l
15163

% beet ls singleton:1|wc -l    
0

% beet ls singleton:true|wc -l
14345

[sampsyo]

Good point—what's missing here, FWIW, is a dbcore type declaration for the singleton field. We don't yet have type declarations for the two built-in "computed" fields on Item: namely, singleton and filesize. It hopefully shouldn't be too hard to add these declarations.