Conform CircularBuffer to ExpressibleByArrayLiteral (#1102)

Motivation:
Making CircularBuffer conform to ExpressibleByArrayLiteral enables
directly initializing it from an array literal, which can sometimes
make its use simpler and more natural, for instance:
foo.circularBuffer = [1,2,3] // vs `= CircularBuffer([1,2,3])`
Foo(circularBuffer: [a, b, c]) // vs `: CircularBuffer([a, b, c])`
This is consistent with other standard Swift collections, such as
Set, as well as existing NIO types (eg: WebSocketMaskingKey).

Modifications:
Extend CircularBuffer to conform to ExpressibleByArrayLiteral

Result:
CircularBuffer can now be initialized from array literals

Sources/NIO/CircularBuffer.swift

@@ -706,3 +706,9 @@ extension CircularBuffer: Hashable where Element: Hashable {
         }
     }
 }
+
+extension CircularBuffer: ExpressibleByArrayLiteral {
+    public init(arrayLiteral elements: Element...) {
+        self.init(elements)
+    }
+}

Tests/NIOTests/CircularBufferTests+XCTest.swift

@@ -75,6 +75,7 @@ extension CircularBufferTests {
                 ("testModify", testModify),
                 ("testEquality", testEquality),
                 ("testHash", testHash),
+                ("testArrayLiteralInit", testArrayLiteralInit),
            ]
    }
 }

Tests/NIOTests/CircularBufferTests.swift

@@ -971,4 +971,17 @@ class CircularBufferTests: XCTestCase {
         XCTAssertEqual(Set([prependBuff,appendBuff]).count, 1)
     }
     
+    func testArrayLiteralInit() {
+        let empty: CircularBuffer<Int> = []
+        XCTAssert(empty.isEmpty)
+        
+        let increasingInts: CircularBuffer = [1, 2, 3, 4, 5]
+        XCTAssertEqual(increasingInts.count, 5)
+        XCTAssert(zip(increasingInts, 1...5).allSatisfy(==))
+        
+        let someIntsArray = [-9, 384, 2, 10, 0, 0, 0]
+        let someInts: CircularBuffer = [-9, 384, 2, 10, 0, 0, 0]
+        XCTAssertEqual(someInts.count, 7)
+        XCTAssert(zip(someInts, someIntsArray).allSatisfy(==))
+    }
 }