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/bistromathic.test

@@ -18,47 +18,64 @@
 cat >input <<EOF
 1+2*3
 EOF
-run 0 7
+run 0 '> 1+2*3
+7
+> '
 
 cat >input <<EOF
 (1+2) * 3
 EOF
-run 0 9
-run -noerr 0 9 -p
+run 0  '> (1+2) * 3
+9
+> '
+run -noerr 0 '> (1+2) * 3
+9
+> ' -p
 
 cat >input <<EOF
 a = 256
 sqrt (a)
 EOF
-run 0 '256
-16'
+run 0 '> a = 256
+256
+> sqrt (a)
+16
+> '
 
 cat >input <<EOF
 a = .16
 b = 10 ^ 2
 sqrt (a * b)
 EOF
-run 0 '0.16
+run 0 '> a = .16
+0.16
+> b = 10 ^ 2
 100
-4'
+> sqrt (a * b)
+4
+> '
 
 cat >input <<EOF
 *
 EOF
-run 0 'err: 1.1: syntax error: expected end of file or - or ( or end of line or double precision number or function or variable before *'
+run 0 '> *
+> err: 1.1: syntax error: expected end of file or - or ( or exit or double precision number or function or variable before *'
 
 cat >input <<EOF
 1 + 2 * * 3
 EOF
-run 0 'err: 1.9: syntax error: expected - or ( or double precision number or function or variable before *'
+run 0 '> 1 + 2 * * 3
+> err: 1.9: syntax error: expected - or ( or double precision number or function or variable before *'
 
 cat >input <<EOF
 100%
 EOF
-run 0 '100
-err: 1.4: error: invalid character'
+run 0 '> 100%
+100
+> err: 1.4: error: invalid character'
 
 cat >input <<EOF
 1 / 0
 EOF
-run 0 'err: 1.1-5: error: division by zero'
+run 0 '> 1 / 0
+> err: 1.1-5: error: division by zero'