Added hexToBytes() String extension

Added two tests to compare Array<UInt8>.init(hex) and hex.hexToBytes()

Sources/CryptoSwift/String+Extension.swift

@@ -58,4 +58,35 @@ extension String {
     public func authenticate<A: Authenticator>(with authenticator: A) throws -> String {
         return try self.utf8.lazy.map({ $0 as UInt8 }).authenticate(with: authenticator).toHexString()
     }
+    
+    
+    private static var unicodeHexMap:[UnicodeScalar:UInt8] = ["0":0,"1":1,"2":2,"3":3,"4":4,"5":5,"6":6,"7":7,"8":8,"9":9,"a":10,"b":11,"c":12,"d":13,"e":14,"f":15]
+    /**
+     Converts the a hexadecimal string to a corresponding array of bytes.
+     
+     So the string "ff00" would get converted to [255, 0].
+     
+     - Returns: An array of 8 bit unsigned integers (bytes).
+     */
+    public func hexToBytes() -> [UInt8]{
+        var bytes:[UInt8] = []
+        var buffer:UInt8?
+        var skip = self.hasPrefix("0x") ? 2 : 0
+        for char in unicodeScalars.lazy {
+            guard skip == 0 else {skip -= 1;continue}
+            guard let value = String.unicodeHexMap[char] else {return []}
+            if let b = buffer {
+                let byte = b << 4 | value
+                bytes.append(byte)
+                buffer = nil
+            } else {
+                buffer = value
+            }
+        }
+        if let b = buffer{
+            bytes.append(b)
+        }
+        return bytes
+    }
+    
 }

Tests/CryptoSwiftTests/ExtensionsTest.swift

@@ -62,6 +62,27 @@ final class ExtensionsTest: XCTestCase {
         let hex = array.toHexString()
         XCTAssertEqual(str, hex)
     }
+    
+    func testArrayInitHexPerformance(){
+        var str = "b1b2b3b3b3b3b3b3b1b2b3b3b3b3b3b3"
+        for _ in 0...9{
+            str += str
+        }
+        measure {
+            let _ = Array<UInt8>(hex: str)
+        }
+    }
+    
+    func testhexToBytesPerformance(){
+        var str = "b1b2b3b3b3b3b3b3b1b2b3b3b3b3b3b3"
+        for _ in 0...9{
+            str += str
+        }
+        measure {
+            let _ = str.hexToBytes()
+        }
+        
+    }
 }
 
 #if !CI