Abstract function declarations should be covered by a set of concrete function declarations in a component.

[sukyoungryu]

This presently (r3029) causes the interpreter to loop infinitely while trying to find the fixed point of imports. We should either report the error properly or fix the infinite loop, as the latter is nigh-impossible to trace (and you can't figure out what was missing when you do).

See source:trunk/ProjectFortress/not_passing_yet/XXXUseMissingExport.fss for a test that ought to fail, but presently passes.