Adding _logChanges()

[DandyLyons]

Related: #2969

In the near future, I would like to implement a _logChanges() function. Using Apple's Logger has a number of benefits over print including:

1. "Jump to line" from the console log.
2. Every log automatically contains helpful metadata, such as the Library and subsystem that created it.
3. Log will automatically protect sensitive user data.
4. You can log different types of messages which will even display in different colors in Xcode.
   See OSLogType.

The Logger object has slightly more upfront work than using print. You cannot simply log. You must first create a Logger, and then you call log on that Logger. Loggers are easy to create, and a Logger can even be labelled with a subsystem and category. With TCA we could potentially add this subsystem and category automatically. For example, we could simply use the name of the Reducer as the subsystem, and the category could be derived from the action. For example, a .viewAction could create a category of View. It's conceivably possible that _logChanges() could handle all of this for us!

Potential Challenges

Unlike print(), the Logger does not accept a String value to log messages. Instead it accepts an OSLogMessage. In practice, this is used almost exactly like a String, however there are certain things you can't do. For example, this doesn't work:

let myString = "Hello world"
let logger = Logger(subsystem: "My Subsystem", category: "My Category")
logger.info(myString) // this won't compile

AFAICT, Apple designed it this way to prevent developers from using Logger to leak user data. Usage is simple though. You just have to define your log messages statically, inline. (String interpolation is still allowed.)

let myString = "world"
let logger = Logger(subsystem: "My Subsystem", category: "My Category")
logger.info("Hello \(myString)")