Redesign VarReference

The current VarReference object contains as only attribute ref that is a reference to a Variable. A Variable object is uniquely initialized with a parameter attribute that can be various objects. It is better to have a direct reference to the actual parameter object rather that to the Variable object containing that object.

Currently this is:

GeneralVariable:
  ObjectImport | Dummy | Variable
;

VarReference:
  !Keyword
  ref = [GeneralVariable|FQN|^args, statements, ~statements.names,
                             ~statements.variables, ~statements.false_,
                             ~statements.true_]
;

The VarReference should be reduced to the cases where we really cannot further resolve the variable, for example for Dummy identifiers. In addition, ~statements.names, ~statements.variables is for tuple imports and tuple variables for which we should always have "normal" imports and variables. It should be checked whether and why we need ~statements.false_, ~statements.true_

This rule will match either a Dummy from the local scope, or FunctionDefinition, ObjectImport defined in the global scope:

GeneralIdentifier:
  FunctionDefinition | ObjectImport | Dummy
;

GlobalScopeReference:
  ref = [GeneralIdentifier|FQN|^args, statements]
;

In the following code all three identifier rules, defined above, will match:

import sqrt from math
func3_2(x) = x*sqrt(x)
print(func3_2(4)) // result is 8

And the following rule will match a Series object that is either an element in columns attribute of a Table object that is in turn a parameter in a top-level Variable object or a parameter attribute in Variable object from the global scope:

SeriesReference:
  ref = [Series|FQN|statements.~parameter.~columns, statements.~parameter]
;

Example for the latter:

ser = (letters: 'a', 'b')
c = ser

Here, the variable c will contain as parameter a SeriesReference object with a reference to the Series object (letters: 'a', 'b') and contain no information about the variable ser.