Prevent redefining bindings by accident. There are
a few cases where we want to allow this, such as the `default` macro, so
we allow a keyword :shadow to be included in the `def` expression to
turn off this lint.
TODO:
* Clean up test suite to remove binding shadowing
* Make sure that we don't get lints with *redef* turned on
* Add positive and negative tests for lint messages.
* Add location of shadowed binding in message
Calvin Rose
c63c6740d9