LKToND introduces excluded middle for equality-left rule

[Gabriel Ebner]

val lk = ProofBuilder.
  c( LogicalAxiom( hof"P(a)" ) ).
  u( WeakeningLeftRule( _, hof"a=b" ) ).
  u( EqualityLeftRule( _, hof"a=b", hof"P(a)", hof"P(b)" ) ).
  qed
val nd = LKToND( lk )

Now nd contains excluded middle (unnecessarily). We should probably handle this using the usual →-introduction+elimination trick.

[Matthias Schlaipfer]

The same with DefinitionLeftRule. Both fixed in e76b24e.