a68: support FFI with C via formal holes

This commit adds FFI support to access C variables and call C
functions from Algol 68, using the construct specified by the Modules
and Separated compilation facilities intended to that effect: formal
holes (`nest' constructs).

Briefly put, an `int foo' C variable can be accessed from Algol 68
like:

  int foo = nest C "foo";
  if foo > 10 then ... fi;

A ref to a C variable `int counter', that allows to modify it as well
as read it:

  ref int counter = nest C "&counter";
  counter +:= 1;

The address stored in an `int *ptr' variable, as an Algol 68
reference:

  ref int ptr = nest C "ptr";
  ptr := 100; { Changes the integer pointed by the C ptr }

Calling the standard C `random' function:

  proc long int random = nest C "random";
  if random < 100 then ... fi;

Some tests and an sketchy section in the manual are also included.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>

gcc/algol68/ChangeLog

	* ga68.vw: Fix grammar for formal holes.
	* ga68.texi (Particular programs): New section.
	(Comments): Likewise.
	* a68.h: Prototypes for a68_is_c_mode, a68_make_fomal_hole_decl,
	a68_lower_formal_hole.
	* a68-parser-attrs.def (FORMAL_HOLE): New attribute.
	(LANGUAGE_INDICANT): Likewise.
	* a68-parser-extract.cc (a68_elaborate_bold_tags): Elaborate
	language indicants.
	* a68-parser-moids-check.cc (mode_check_unit): Check mode in
	formal holes.
	* a68-low-units.cc (a68_lower_formal_hole): New function.
	* a68-low.cc (a68_make_formal_hole_decl): Likewise.
	* a68-moids-misc.cc (a68_is_c_mode): Likewise.
	* a68-parser-bottom-up.cc (reduce_formal_holes): Likewise.

gcc/testsuite/ChangeLog

	* algol68/compile/error-nest-1.a68: New test.
	* algol68/compile/nest-c-1.a68: Likewise.
	* algol68/compile/error-nest-5.a68: Likewise.
	* algol68/compile/error-nest-4.a68: Likewise.
	* algol68/compile/error-nest-3.a68: Likewise.
	* algol68/compile/error-nest-2.a68: Likewise.

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