Add location info to MacroVar nodes

[Blacksmoke16]

This manifested itself as:

macro test(val)
  {% if val == 1 %}
    %a = {{val.id}}
  {% else %}
    %a = {{val.id}}
  {% end %}
end

test 1
test 2

having reported coverage of:

{
  "coverage": {
    "test.cr": {
      "2": 1,
      "3": 1,
      "4": 0,
      "5": 1
    }
  }
}