Add support for earlier OS versions + Change stream line separator to CRLF

[MMP0]

Lower the minimum supported OS versions to macOS 10.15 and iOS 13, the lowest versions that support Swift Concurrency.

Also I noticed an issue while creating a polyfill: the Twitter developer page mentions that the line separator in the streaming API is \r\n, but AsyncLineSequence interprets CR (\r), LF (\n), CRLF (\r\n), NEL (\u{85}), LS (\u{2028}) and PS (\u{2029}) as line separators. This could potentially cause problems with tweets containing these characters. So I created AsyncCRLFLineSequence.

[capnslipp]

Personally, I would put extensions & polyfills each in their own file based on what type they're extending, e.g. DateExtensions.swift (common Swift style), Date+TwiftExtensions.swift (older Obj-C style), or following the existing naming of files in Twift, Date++.swift , or maybe Date+Polyfill.swift considering the use-case.

[capnslipp]

You can wrap functions & structs in available (by using @available instead of #available), so they can have the same name as Apple's identifiers, but not conflict with them on newer OSes.

E.G. Change this:

extension Date {
  func _ISO8601Format() -> String {
    if #available(iOS 15.0, macOS 12.0, *) {
      …
    }
  }
}

to this:

@available(iOS 15.0, macOS 12.0, *)
extension Date {
  func ISO8601Format() -> String {
    … // only your custom implementation here; no need to call out to `ISO8601Format()`
  }
}

[capnslipp]

Likewise for AsyncBytes regarding the @available(…) attribute comments above.

[MMP0]

Thank you for pointing those out. I’ve fixed them.