AST: remove redundant assertions for error tolerance.

* da/compiler/dast.py(DomainSpec.pattern): remove assertion.
Fix exception when parsing malformed patterns in history expressions.

* da/compiler/parser.py(Parser.event_from_pattern): don't assert
  TuplePattern, check.
Fix wrong tuple generated for history expressions.

* da/compiler/parser.py(Parser.parse_domain_spec): back patch pattern
  for "xxx in received/sent" expressions.