FreeSpec.Exec may fail to execute terms with opaque parts

[vtourneur]

When executing a Program term, FreeSpec.Exec reduces it with Reduction.whd_all. If this term contains some opaque parts, this may not lead to a term begining with a constructor of Program, which gives the following error message: UnsupportedTerm "FreeSpe.Exec only handles [FreeSpec.Program] terms." (despite that the term is of type Program). This is in particular the case with Program Fixpoint defined terms.