ada.2: spring cleaning before final pull request.

ada.2: spring cleaning before final pull request.

Two changes require approval.

* The 'do' special becomes a built-in function similar to first. This
  small change reduces the complexity of eval.  The last evaluation
  cannot benefit from TCO, but the performance change seems invisible.

* read/eval/print acts on each item found in the input string, as if
  they were enclosed with (do ..). The guide does not specify what
  should happen to text following the first AST, and this change actually
  simplifies some things (like dealing with zero AST).
  The read-string built-in function only returns the first AST,
  as changing this would be much more intrusive.

Other changes seem straightforward.

Global:
* Ada 2020 target assignments (like +=, but more general).
* Use Constant_Indexing aspect for sequences, so that they can be
  indexed in source code like native arrays.
* consistency renamings.
  'fn' does not include built-in functions, 'function' does.
  'list' does not include vectors, 'sequence' does.

Move error handling to a separate package.
* This simplifies code everywhere else.
* Uncaught expressions now report a stack trace.

Types:
* Count allocations and deallocations, check that counts match.
* Share more code between functions and macros.

Core:
* Replace the Core.Ns function returning an array with a procedure
  (The intermediate object was preventing the reference counting code
  from deallocating some unused objects).
* Implement Prn with Pr_Str.

Printer:
* Change the profile so that the caller spares some allocations.

Reader:
* Share a single buffer of mal values between all recursions.
  This significantly reduces the stack footprint.

Steps:
* Fix implementation name (ada.2) in the startup script.
* Let environment variables trigger debugging information.