examples: add an example of a push parser

Add an example to demonstrate the use of push parser.  I'm pleasantly
surprised: parse.error=detailed works like a charm with push parsers.

* examples/c/local.mk, examples/c/pushcalc/Makefile
* examples/c/pushcalc/README.md, examples/c/pushcalc/calc.test,
* examples/c/pushcalc/calc.y, examples/c/pushcalc/local.mk:
New.

examples/c/pushcalc/README.md

@@ -0,0 +1,39 @@
+# pushcalc - push parser with Bison
+
+This directory contains pushcalc, the traditional calculator, implemented as
+a push parser.
+
+Traditionally Bison is used to create so called "pull parsers": the user
+invokes the parser once, which repeatedly calls (pulls) the scanner until
+the input is drained.
+
+This example demonstrates the "push parsers": the user calls scanner to
+fetch the next token, passes (pushes) it to the parser, and repeats the
+operation until the input is drained.
+
+This example is a straightforward conversion of the 'calc' example to the
+push-parser model.
+
+<!---
+Local Variables:
+fill-column: 76
+ispell-dictionary: "american"
+End:
+
+Copyright (C) 2020 Free Software Foundation, Inc.
+
+This file is part of Bison, the GNU Compiler Compiler.
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+--->