The implementation of `hy.core.language.exec` draws code from the `exec_` function in commit f574c7be6ebc80041ef58ca29588f310248ebed4 of the library Six, which is copyright 2010–2017 Benjamin Peterson and licensed under the Expat license.
This cleans up a number of doc warnings, including a bad
underline for zero?
While there, added a nil? function to match up with the
new nil is None.
Also un-hid myself from coreteam.
