Implement IR printing functionality of the PassManager

[bitwalker]

During development of the new IR, I stubbed out functionality for enabling the PassManager implementation to print the IR before/after each pass is applied, optionally only when modified, etc.

As the size of the programs we're troubleshooting grows, it will become increasingly necessary to be able to filter tracing/debug output to specific functions or operations. As a temporary workaround, I recently added the Print pass in #454 that provides this to a limited degree, but what we really need is for the PassManager to provide this for us, as it can provide much more precision.

The configuration and enablement of the feature is partially implemented, but the main thing missing is the actual handling of that configuration at appropriate points during pass pipeline execution.

We likely also need to come up with a better wrapper around the log crate, so we can emit more precise log targets, enabling us to filter the log output by function and target, but I'll defer that to another issue.

[mooori]

I'm giving this a try in #486. Even though it's still a draft, could you perhaps have a quick look and let me know if it's going in the right direction?