Remove linked-in ABC

[atomb]

Once #562 is implemented, but as part of a later release, we should remove the linked-in version of ABC to avoid some of the build issues and crashes that it has led to. This would resolve issues #156, and #233.

[atomb]

My current plan is to finish adding alternatives to all SAW commands based on the linked-in ABC but then do the actual removal of ABC from the build process after the 0.8 release. That way 0.8 will include both the linked-in and external-process versions so they can readily be compared to provide input into improvements needed before 0.9.

[atomb]

PR #1320 implements most of the abcBridge-based commands with pure Haskell code (the exception being cec, which we haven't yet implemented but could). Merging that PR will close this issue.