Leverage github actions for tests and linting.
This includes fixing a bunch of issues highlighted by golangci
including:
* Dead code.
* Ineffectual assigns.
* Goto warnings.
* Nil return err.
* Reused literal strings.
* Test parameter order.
Also:
* Setup clog.
* (#257) Change Walk/Visit to follow an explicit Enter/Exit pattern
* (#257) Convert walk example into test.
* (#257) restore walk/visitor example
* (#257) Fix godoc comment in the Visitor interface
... typo referred to `End` method instead of `Exit`
* Streamline what we get from "otto/parser"
* Get rid of some "otto/parser" cruft
* FunctionExpression => FunctionLiteral
* The debugger statement (debugger) should do nothing (not panic)
* Fix aspects of function expression call evaluation
* Faster, more straightforward, etc.
* More advanced object literals (get ..., set ...)
* More tests using JavaScript from the wild (http://cdnjs.com/)
