Parser: register 'del' as assignments instead of updates.

Deleting a variable is changing its alias, hence 'del' statements should
be assignments to the variable, instead of updates.

* da/compiler/parser.py(Parser): register 'Delete' contexts as
  assignment to NamedVar; change context to 'Update' when 'Delete'
  context on fields or slices.
AST: implement 'nameobjs' property for statements.

* da/compiler/dast.py: implement 'nameobjs' property for statements.
Update language spec.
Revert to 'sub' semantics by default.

* da/compiler/parser.py:
  -(PatternParser.is_bound, PatternParser.is_free): check current_query
  instead of top_level_query scope, unless 'use_top_semantic'.
  -(Parser.audit_query): no need to audit query if not using 'top'.

* da/compiler/pygen.py(PythonGenerator):
  -(__init__): new argument 'options', for command line options.
  -(get_option): new method, copied from 'Parser.get_option'.
  -(visit_Program): save module '_compiler_options'.
  -(visit_QuantifiedExpr, visit_ComprehensionExpr): reset pattern
  generator state for nested queries, unless 'use_top_semantic'.

* da/compiler/ui.py
  -(dafile_to_pyast, dastr_to_pyast): pass command line options to
   'PythonGenerator'.
  -(main): new command line option '--use-top-semantic'.
Bump version to 1.0.0b12.