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/README.md

@@ -50,9 +50,13 @@ This example is a straightforward conversion of the 'calc' example to the
 push-parser model.
 
 ## bistromathic - all the bells and whistles
-This example demonstrates the best practices when using Bison.
-- Its interface is pure.
+This example demonstrates best practices when using Bison.
 - Its hand-written scanner tracks locations.
+- Its interface is pure.
+- Its interface is "incremental", well suited for interaction: it uses the
+  push-parser API to feed the parser with the incoming tokens.
+- It features an interactive command line with completion based on the
+  parser state, based on `yyexpected_tokens`.
 - It uses a custom syntax error with location, lookahead correction and
   token internationalization.
 - It supports debug traces with semantic values.