Allow par identifiers var array initialisers for FlatZinc

[cyderize]

Previously, the parser would reject par identifiers inside the initializers for arrays of var declarations, but this should (theoretically) be allowed (although the MiniZinc compiler has now been changed to never produce this).

Also adds a missing check for restart_data preventing a possible crash.

[cyderize]

Hi @zayenz, we're trying to get a release of MiniZinc out soon, so it would be great if you could have a look at this (to fix MiniZinc/libminizinc#756). Thanks!

[zayenz]

Just for completeness, is there any way that say an int constant would be put in a float variable array?

Otherwise, looks great.

[cyderize]

Thanks for merging! :)

The MiniZinc compiler never puts integer literals in float arrays (and if it did I would consider that a bug in the compiler), but I guess the specification of FlatZinc doesn't really say what a well-typed program looks like - maybe we should explicitly mention that no subtyping is allowed other than par/var.