libcdecl: Rework specifier output logic.

libcdecl: Rework specifier output logic.

With both the explain and declare code paths using cdecl__emit, we
can give the same treatment to cdecl__explain_specs (now called
cdecl__emit_specs) to simplify things a bit.

This removes the last caller of cdecl__advance, so we can remove
that function (and use this better name for cdecl__advance_).