Array: add leftOf / rightOf accessor methods (#18)

* Array: add leftOf / rightOf accessor methods

* Code review feedback, better naming.

Sources/FoundationExtensions/Collection/Array+LeftRightOf.swift

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+//
+//  Array+LeftRightOf.swift
+//  FoundationExtensions
+//
+//  Created by Thomas Mellenthin on 15.09.20.
+//  Copyright © 2018 Lautsprecher Teufel GmbH. All rights reserved.
+//
+
+import Foundation
+
+extension Array where Self.Element : Equatable {
+
+    public func left(of element: Element) -> Element? {
+        guard let elementIndex = firstIndex(of: element) else { return nil }
+        if elementIndex == 0 {
+            return self.last
+        }
+        return self[safe: (elementIndex - 1)]
+    }
+
+    public func right(of element: Element) -> Element? {
+        guard let elementIndex = firstIndex(of: element) else { return nil }
+        if elementIndex >= (count - 1) {
+            return self.first
+        }
+        return self[safe: (elementIndex + 1)]
+    }
+}

Tests/FoundationExtensionsTests/Array+LeftRightOfTests.swift

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+//
+//  ArrayLeftRightOfTests.swift
+//  FoundationExtensionsTests
+//
+//  Created by Thomas Mellenthin on 15.09.20.
+//  Copyright © 2018 Lautsprecher Teufel GmbH. All rights reserved.
+//
+
+#if !os(watchOS)
+import FoundationExtensions
+import XCTest
+
+class ArrayLeftRightOfTests: XCTestCase {
+}
+
+extension ArrayLeftRightOfTests {
+    func testRightOf() {
+        // whe
+        let sut = [1, 3, 7, 9, 25]
+
+        // then
+        XCTAssertEqual(sut.right(of: 1), 3)
+        XCTAssertEqual(sut.right(of: 9), 25)
+        XCTAssertEqual(sut.right(of: 25), 1)
+        XCTAssertEqual(sut.right(of: 666), nil)
+    }
+
+    func testLeftOf() {
+        // whe
+        let sut = [1, 3, 7, 9, 25]
+
+        // then
+        XCTAssertEqual(sut.left(of: 1), 25)
+        XCTAssertEqual(sut.left(of: 3), 1)
+        XCTAssertEqual(sut.left(of: 25), 9)
+        XCTAssertEqual(sut.left(of: 666), nil)
+    }
+}
+#endif