Macroprocessor: replace naked pointers by smart pointers

- use std::unique_ptr for MacroDriver::lexer
- use std::shared_ptr for MacroValue objects constructed by the parser
  This is made possible using Bison 3.0 variant value type.
  Using non-pointer type is not possible since MacroValue is polymorphic.
  Using std::unique_ptr is not possible since the value type must be
  copyable (see
  https://lists.gnu.org/archive/html/bug-bison/2015-03/msg00004.html).
  Define a new type MacroValuePtr == shared_ptr<const MacroValue> for code
  clarity.
  Use the following convention for shared_ptr arguments to functions:
  + if pass-by-value, means that the callee acquires ownership
  + if pass-by-const-reference, the callee does not acquire ownership
- naked pointers are still used for tracking the filename in Bison's location
  type, since we don't have control over that

Also, by the way:
- arrays can now contain elements of any type
- simplify MacroDriver::loop_stack using std::tuple
- toArray() method now fails on function objects
- no need to use const qualifiers, since all MacroValue objects are immutable
- use an exception for detecting division by zero