Crafting a better parser-priming hack.

The method of inserting a character sequence which generates a
SECRET_TOKEN_E token is being replaced with a purely token based
method.

Because we don't manipulate the input stream, the lexer is not
involved. We don't have to flush its state and deal with the carry-over
of the yy_hold_char.

This comes about because recent changes expose a weakness in the old
scheme. Now that a top-level expression can have the form expr.expr, it
means that the Yacc parser reads one token ahead, to see whether there
is a dot or something else.  This lookahead token is discarded. We must
re-create it when we call yyparse again. This re-creation is done by
creating a custom yylex function, which can maintain pushback tokens.
We can prime this array of pushback tokens to generate the
SECRET_TOKEN_E, as well as to re-inject the lookahead symbol that was
thrown away by the previous yyparse.  To know which lookahead symbol to
re-inject is simple: the scanner just keeps a copy of the most recent
token that it returns to the parser. When the parser returns, that
token must be the lookahead one.

The tokens we keep now in the parser structure are subject to garbage
collection, and so we must mark them.  Since the YYSTYPE union has no
type field, a new API is opened up into the garbage collector to help
implement a conservative GC technique.

* gc.c (gc_is_heap_obj): New function.

* gc.h (gc_is_heap_obj): Declared.

* match.c: Include y.tab.h. This is now needed by any module
that needs to instantiate a parser_t structure, because members
of type YYSTYPE occur in the structure. (parser.h can still be included
without y.tab.h, but only an incomplete declaration for the parser
strucure is then given, and a few functions are not declared.)

* parser.c (yy_tok_mark): New static function.
(parser_mark): Mark the recent token and the pushback tokens.
(parser_common_init): Initialize the recent token, the
pushback tokens, and the pushback stack index.
(pushback_token): New static function.
(prime_parser): hold_byte argument removed. Body considerably
simplified. The catenated stream trick is no longer required.
All we do here is set up two pushback tokens and prime the scanner,
if necessary, so it is in the right start state for Lisp.

* parser.l (YY_DECL): Take over definition of scanning function, renaming
to yylex_impl, so we can implement yylex.
(grammar): Rule which produces SECRET_ESCAPE_E token removed.
(reset_scanner): Function removed.
(yylex): New function.

* parser.h (struct parser): Now only forward-declared unless y.tab.h
has been included. New members, recent_tok, tok_pushback and tok_idx.
(yyset_hold_char): Declared.
(reset_scanner): Declaration removed.
(yylex): Declared (if y.tab.h included).
(prime_parser): Declaration updated.
(prime_scanner): Declared.

* Makefile: express new dependency on existence of y.tab.h of txr.o,
match.o and parser.o.

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