assert: use util.inspect() to create error messages

Currently, JSON.stringify() is used to create error messages
on failed assertions. This causes an error when stringifying
objects with circular references. This commit switches out
JSON.stringify() for util.inspect(), which can handle
circular references.

PR: #8734
PR-URL: https://github.com/joyent/node/pull/8734


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>