Fix: setConstant and setModuleConstant now function correctly again.

Previously, __filename, __dirname, and other constants were broken. This was due to
the refactor of setConstant and setModuleConstant, and the reuse of the variable name "expr"
Merge pull request #342 from Taytay/fix/node_stuff_set_constants

Fix: setConstant and setModuleConstant now function correctly again.