Remove unsafe code Fixes #7, #8

The traits `Send + Sync` don't need to be implemented manually since
they get automatically implemented. To make sure this stays that way
a test was added.

This Commit also Removes a warning while generating the documentation.
```
warning: unresolved link to `RFC2234`
   --> src/lib.rs:220:36
    |
220 | > and is expressed in the ABNF of [RFC2234].
    |                                    ^^^^^^^ no item named `RFC2234` in scope
    |
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
    = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: `email_address` (lib doc) generated 1 warning
```

src/lib.rs

@@ -217,7 +217,7 @@ These in turn refer to RFC 6529 §4. [Syntax of UTF-8 Byte Sequences](https://to
 > A UTF-8 string is a sequence of octets representing a sequence of UCS
 > characters.  An octet sequence is valid UTF-8 only if it matches the
 > following syntax, which is derived from the rules for encoding UTF-8
-> and is expressed in the ABNF of [RFC2234].
+> and is expressed in the ABNF of \[RFC2234\].
 
 ```ebnf
    UTF8-octets = *( UTF8-char )
@@ -390,12 +390,6 @@ impl Display for Error {
     }
 }
 
-#[allow(unsafe_code)]
-unsafe impl Send for Error {}
-
-#[allow(unsafe_code)]
-unsafe impl Sync for Error {}
-
 impl std::error::Error for Error {}
 
 impl<T> From<Error> for std::result::Result<T, Error> {
@@ -1086,4 +1080,14 @@ mod tests {
     fn test_bad_example_04() {
         expect("simon@", Error::DomainEmpty, Some("domain is empty"));
     }
+
+    // make sure Error impl Send + Sync
+    fn is_send<T: Send>() {}
+    fn is_sync<T: Sync>() {}
+
+    #[test]
+    fn test_error_traits() {
+        is_send::<Error>();
+        is_sync::<Error>();
+    }
 }