[SR-370] Remove special case Array.extend() for collections

[gribozavr]

Previous ID	SR-370
Radar	rdar://problem/21761471
Original Reporter	@gribozavr
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug, AffectsABI, Performance
Assignee	@airspeedswift
Priority	Medium

md5: 1975b6365d6939725f86c751ae2e749c

Issue Description:

SVN r30033 added a special case of Array.extend() (now appendContentsOf) for values that are statically typed as collections. What if the value is typed statically as a sequence (e.g., in generic code), but is dynamically a collection? This method wouldn’t apply, and we have lost performance.

We need to thread this through the generics system properly.

[airspeedswift]

This ought to be resolved by the code to implement SE-147 found here (though that's still a WIP)

[bob-wilson]

Ben: "This has been fixed, the collection-specific variant for append (as it’s now called) is gone from master."