Fix macro so it doesn't require :refer'd var

The debug-repl macro currently expands to '(debug-repl nil) when called
with zero args, which only works if the user has done
`(use 'alex-and-georges.debug-repl)` (or equivalent) as instructed in
the README. There's no reason to require this, so changing the zero-arg
version of the macro to expand to the fully-qualified symbol is better
macro-writing practice and prevents confusion for people not wanting to
refer the var.