Fix breakage of abstraction introduced by commit 2923f216.
That commit caused the declaration list to no longer be reset after each declaration. Fixing this issue speeds up Gappa printing by 800%. Abstracted subterms and formulas are now systematically added to extra_decls, since their presence in the global tables does not guarantee they have already been defined. This guarantee was lost when the tables became global to the transformation, but it was hidden by list extra_decls never being emptied. Note: removing alpha-conversion does not bring any sensible speedup.
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