workaround failed associated type inference (#1299)
workaround failed associated type inference (#1299)

Motivation:

Some Swift nightly snapshots fail to compile NIO in release mode
(rdar://57793267), this change is to work around this issue by telling
them compiler the associatedtypes explicitly.

Modifications:

- add `Element` and `Indices` typealises to `CircularBuffer` explicitly.

Result:

Complies on all nightlies also in release mode.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio