refactor: use Swift CommonCrypto module to calculate md5 hashes (#22)

Sources/ImgixClient.swift

@@ -150,7 +150,7 @@ import Foundation
             signatureBase += "?" + queryString
         }
 
-        let signature = signatureBase.ixMd5
+        let signature = signatureBase.ixMd5()
 
         return URLQueryItem.init(name: "s", value: signature)
     }

Sources/String+ImgixSwift.swift

@@ -7,6 +7,9 @@
 //
 
 import Foundation
+import var CommonCrypto.CC_MD5_DIGEST_LENGTH
+import func CommonCrypto.CC_MD5
+import typealias CommonCrypto.CC_LONG
 
 extension String {
     static var ixEncodeUriComponentCharSet: CharacterSet = {
@@ -32,4 +35,25 @@ extension String {
     func ixEncodeUriComponent() -> String {
         return self.addingPercentEncoding(withAllowedCharacters: String.ixEncodeUriComponentCharSet)!
     }
+
+    func ixMd5() -> String {
+        return MD5(string: self)
+    }
+
+    func MD5(string: String) -> String {
+        let length = Int(CC_MD5_DIGEST_LENGTH)
+        let messageData = string.data(using:.utf8)!
+        var digestData = Data(count: length)
+
+        _ = digestData.withUnsafeMutableBytes { digestBytes -> UInt8 in
+            messageData.withUnsafeBytes { messageBytes -> UInt8 in
+                if let messageBytesBaseAddress = messageBytes.baseAddress, let digestBytesBlindMemory = digestBytes.bindMemory(to: UInt8.self).baseAddress {
+                    let messageLength = CC_LONG(messageData.count)
+                    CC_MD5(messageBytesBaseAddress, messageLength, digestBytesBlindMemory)
+                }
+                return 0
+            }
+        }
+        return digestData.map { String(format: "%02hhx", $0) }.joined()
+    }
 }