Updated Review Status
The core team has resolved to change the status of this proposal to accepted with modification, deferring the .isEmoji property to a later proposal.
The core team still believe it is a useful feature for the standard library, but needs more investigation that should not hold up the rest of this proposal.
In addition, the proposal will drop the source-breaking change to the existing FixedWidthInteger.init?. While this would be the preferred naming for a newly proposed initializer, it doesn't clear the bar for a source-breaking change for Swift 5.