mal/ada.2

Comparison with the first Ada implementation.
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The first implementation was deliberately compatible with all Ada
compilers, while this one illustrates various Ada 2020 features:
assertions, preconditions, invariants, initial assignment for limited
types, limited imports, indexing aspects...

The variant MAL type is implemented with a discriminant instead of
object-style dispatching.  This allows more static and dynamic checks,
but also two crucial performance improvements:
* Nil, boolean, integers and pointers to built-in functions are passed
  by value without dynamic allocation.
* Lists are implemented as C-style arrays, and most of them can be
  allocated on the stack.

Another difference is that a minimal form of garbage collecting is
implemented, removing objects not referenced from the main
environment. Reference counting is convenient for symbols or strings,
but never deallocates cyclic structures. The implementation collects
garbage after each Read-Eval-Print cycle. It would be much more
difficult to collect garbage inside scripts. If this is ever done, it
would be better to reimplement load-file in Ada and run a cycle after
each root evaluation.

The eventual performances compete with C-style languages, allthough
all user input is checked (implicit language-defined checks like array
bounds and discriminant consistency are only enabled during tests).

Notes for contributors that do not fit in a specific package.
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* All packages can call Eval back via a reference in the Eval_Cb
  package, set during startup. I am interested in a prettier solution
  ensuring a valid value during elaboration.
  Note that generic packages cannot export access values.

* Symbol pointers are non null, new variables must be assigned
  immediately. This is usually enforced by a hidden discriminant, but
  here we want the type to become a field inside Types.Mal.T. So the
  check happens at run time with a private invariant.

  The finalize procedure may be called twice, so it does nothing when
  the reference count is zero, meaning that we are reaching Finalize
  recursively.

* In implementations with reference counting, a consistent object
  (that will be deallocated automatically) must be built before any
  exception is raised by user code (for example the 'map' built-in
  function may run user code).  Garbage collection simplifies a lot
  this kind of situations.

* Each module encapsulating dynamic allocation counts allocations and
  deallocations. With debugging options, a failure is reported if
  - too many deallocation happen (via a numeric range check)
  - all storage is not freed (via a dedicated call from the step file)

  The main program only checks that the garbage collector removes all
  allocations at the end of execution.

Debugging
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Uncaught exceptions are reported with an execution trace (excluding
TCO cycles).  This has become possible in step9, but has been
backported to former steps as this is really handy for debugging.

Some environment variables increase verbosity.
# dbgread= ./stepAmal     trace reader recursion
# dbgeval= ./stepAmal     trace eval recursion (including TCO)