AST: add type checks for 'DomainSpec'.
AST: add type checks for 'DomainSpec'.

* da/compiler/dast.py:

  -(DomainSpec.ordered_boundvars, DomainSpec.ordered_freevars): check
   whether 'self.pattern' is PatternExpr.

sghosh1991/distalgo,sghosh1991/distalgo