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/README.md

@@ -37,6 +37,17 @@ recursively.  To demonstrate this feature, expressions in parentheses are
 tokenized as strings, and then recursively parsed from the parser.  So
 `(((1)+(2))*((3)+(4)))` uses eight parsers, with a depth of four.
 
+## pushcalc - calculator implemented with a push parser
+All the previous examples are so called "pull parsers": the user invokes the
+parser once, which repeatedly calls the scanner until the input is drained.
+
+This example demonstrates the "push parsers": the user calls the scanner to
+fetch the next token, passes 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.
+
 
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