Add tests for pushnew errors.

Currently pushnew allows multiple evaluation and not altering every
possible place
Fix append on abstract sequences.

It was returning nil instead of a new sequence.
Fix pushnew.

It was suffering from multiple evaluation and failure to update some
places.
Add tests for multiple evaluation in push and pop.
Prevent multiple evaluation in push and friends.

Push, push-back, pushnew, pushnew-back, and pop were affected.
Merge pull request #49 from m-n/abstract-sequence-setf

Push and Pop multiple evaluation