[SR-1428] Add default argument to `Sequence.joined(separator:)` method

[benrimmington]

Previous ID	SR-1428
Radar	None
Original Reporter	@benrimmington
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Improvement
Assignee	None
Priority	Medium

md5: a5f63c1a862ff9e824a317866e8a345b

Issue Description:

A joined sequence of sequences could use an EmptyCollection as the default separator.

A joined sequence of strings could use an empty string as the default separator.

  1> ["con", "caten", "ated"].joined()
$R0: String = "concatenated"
  2> ["hyphen", "ated"].joined(separator: "-")
$R1: String = "hyphen-ated"

[benrimmington]

https://github.com/apple/swift-evolution/blob/master/proposals/0133-rename-flatten-to-joined.md

[benrimmington]

Resolved by:

• [stdlib] Rename flatten() to joined() (SE-0133) #3809

• [stdlib] default separator for joining strings (SE-0133) #3838

• [stdlib] Update doc comments for SE-0133 #3839