add Principal.fromBlob (#331)

Author: chenyan-dfinity

src/Principal.mo

@@ -15,6 +15,9 @@ module {
   /// Conversion.
   public let toBlob : (p : Principal) -> Blob = Prim.blobOfPrincipal;
 
+  /// Conversion.
+  public let fromBlob : (b : Blob) -> Principal = Prim.principalOfBlob;
+  
   /// Conversion.
   public func toText(p : Principal) : Text = debug_show(p);