Consider whether to be using fatalErrors vs precondition/preconditionFailures

precondition and preconditionFailure are not evaluated in unchecked builds ( -Ouncheck), but fatalErrors are.

we pretty much just use precondition/preconditionFailure everywhere. Swift std lib uses a mix of both.

we should further research this and come up with a rule of thumb as to whether to use fatalError or precondition in various places, and correct any existing violations. 

blog posts that seem promising for learning more:

• https://agostini.tech/2017/10/01/assert-precondition-and-fatal-error-in-swift/
• https://blog.krzyzanowskim.com/2015/03/09/swift-asserts-the-missing-manual/ 
• https://www.swiftbysundell.com/posts/picking-the-right-way-of-failing-in-swift