Implemented 'to be a non-empty array whose items satisfy'.
Added 'to be non-empty' as an alias for 'not to be empty' for consistency.
Implemented 'to be an empty object' and 'to be a non-empty object'.
Allow 'object' and 'hash' as aliases of 'map'.
Added support for the 'non-empty' flag in the 'to be a map whose keys satisfy' assertion.
Use the 'to be non-empty' to make the code slightly more readable.
Changed recurring regexp to require exactly three letters instead of three or more. Makes the intention clearer.