a68: more work on formal holes

a68: more work on formal holes

This commit implements several improvements:

1. The optimization for avoiding indirect calls while using
   declarations like:

    proc(string)int puts = nest C "_libga68_posixputs";

   has been completed.

2. Algol 68 procedures getting strings as arguments can now
   wrap corresponding C functions.  Note this does not include
   procedures yielding strings as for now.

3. Wrappers are now built for all formal holes having proc mode.  This
   allows for a more robust implementation.

Signed-off-by: Jose E. Marchesi <jemarch@gnu.org>
gcc/algol68/ChangeLog

	* Make-lang.in (ALGOL68_OBJS): Add algol68/a68-low-holes.o.
	* a68.h: Update prototypes.
	* a68-types.h (struct TAG_T): New field nest_proc.
	(NEST_PROC): Define.
	* a68-parser.cc (a68_new_tag): Initialize NEST_PROC.
	* a68-parser-extract.cc (extract_identities): Use NEST_PROC
	instead of IN_PROC for taxes for defining-identifiers in identity
	declarations of proc modes with formal holes as actual parameters.
	* a68-moids-misc.cc (a68_is_c_mode): Modified to allow strings as
	direct parameters.
	* a68-low.cc (a68_make_proc_formal_hole_decl): Remove.
	* a68-low-units.cc (a68_lower_identifier): Improve commentary.
	(a68_lower_formal_hole): Factorize.
	* a68-low-holes.cc: New file.
	* a68-low-decls.cc (a68_lower_identity_declaration): Optimize
	identity declarations of proc mode with formal holes as actual
	parameters.
	* a68-exports.cc (a68_add_identifier_to_moif): Honor NEST_PROC.
	* ga68.texi (Communicating with C): Strings can now be passed as
	parameters in formal holes.

gcc/testsuite/ChangeLog

	* algol68/compile/error-nest-4.a68: Strings can now be passed as
	arguments in formal holes.