Make the representation of ObjectIdentifier more private.

In preperation for #181

src/object_identifier.rs

@@ -11,7 +11,7 @@ use alloc::vec::Vec;
 #[derive(Debug, PartialEq, Eq, Clone, Hash)]
 pub struct ObjectIdentifier<'a> {
     // Store the OID as DER encoded. This means we can 0-copy on parse.
-    pub(crate) der_encoded: Cow<'a, [u8]>,
+    der_encoded: Cow<'a, [u8]>,
 }
 
 fn _read_base128_int<I: Iterator<Item = u8>>(mut reader: I) -> Option<u32> {
@@ -86,6 +86,10 @@ impl<'a> ObjectIdentifier<'a> {
             der_encoded: Cow::Borrowed(data),
         })
     }
+
+    pub(crate) fn as_der(&self) -> &[u8] {
+        &self.der_encoded
+    }
 }
 
 impl fmt::Display for ObjectIdentifier<'_> {

src/types.rs

@@ -644,7 +644,7 @@ impl<'a> SimpleAsn1Readable<'a> for ObjectIdentifier<'a> {
 impl<'a> SimpleAsn1Writable<'a> for ObjectIdentifier<'a> {
     const TAG: u8 = 0x06;
     fn write_data(&self, dest: &mut Vec<u8>) {
-        dest.extend_from_slice(&self.der_encoded);
+        dest.extend_from_slice(&self.as_der());
     }
 }