Remove deprecated MonadZero instance (v0.15.0 PS) #64
Description
When we make the next round of breaking changes (e.g. v0.15.0 of PureScript), this is a reminder to remove the MonadZero instance for Parser. Relates to #63, which censors user-defined warnings due to the MonadZero instance. The build command should be updated to fail on any warning once we've removed the instance.