examples: bistromathic: demonstrate the use of yyexpected_tokens

Let's use GNU readline and its TAB autocompletion to demonstrate the
use of yyexpected_tokens.

This shows a number of weaknesses in our current approach:

- some macros (yyssp, etc.) from push parsers "leak" in user code, we
  need to undefine them

- the context needed by yyexpected_tokens does not need the token,
  yypstate actually suffices

- yypstate is not properly setup when first allocated, which results
  in a crash of yyexpected_tokens if fired before a first token was
  read.  We should move initialization from yypush_parse into
  yypstate_new.

* examples/c/bistromathic/parse.y (yylex): Take input as a string, not
a file.
(EXIT): New token.
(input): Adjust to work only on a line.
(line): Remove.
(symbol_count, process_line, expected_tokens, completion)
(init_readline): New.
* examples/c/bistromathic/bistromathic.test: Adjust expectations.

examples/c/bistromathic/local.mk

@@ -27,7 +27,7 @@ nodist_%C%_bistromathic_SOURCES = %D%/parse.y %D%/parse.h
 
 # Don't use gnulib's system headers.
 %C%_bistromathic_CPPFLAGS = -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
-%C%_bistromathic_LDADD = -lm
+%C%_bistromathic_LDADD = -lm -lreadline
 
 dist_bistromathic_DATA = %D%/parse.y %D%/Makefile %D%/README.md
 CLEANFILES += %D%/parse.[ch] %D%/parse.output