Add conversion from and to hex for SRPKey

Author: adam-fowler

Sources/SRP/keys.swift

@@ -2,16 +2,22 @@ import BigNum
 
 /// Wrapper for keys used by SRP
 public struct SRPKey {
-    public var bytes: [UInt8] { number.bytes }
     public let number: BigNum
-    
+    public var bytes: [UInt8] { number.bytes }
+    public var hex: String { number.hex }
+
     public init(_ bytes: [UInt8]) {
         self.number = BigNum(bytes: bytes)
     }
 
     public init(_ number: BigNum) {
         self.number = number
     }
+    
+    public init?(hex: String) {
+        guard let number = BigNum(hex: hex) else { return nil }
+        self.number = number
+    }
 }
 
 extension SRPKey: Equatable { }