Disable Goto-abductive prover as target.  Tidied up temporal prover
files that were already disabled.
FO prover changed to do "explicit" back-linking.
Improved FO prover to retain butterfly-style proofs for odd-or-even is
nat.
Goto prover now uses explicit backlinking via
folding/weakening/subtitutions.
While regular and abductive provers now use explicit backlinking.
Core proof-manipulation functions now enforce back-linking only between
syntactically equal sequents.  Some documentation added.