message: Format: Make v1 magic explicitly separate from age magic
  From the specification:
  > The first line of the header is age-encryption.org/ followed by an
  > arbitrary version string.
diff: @@ -10,7 +10,8 @@ pub(crate) mod ssh_ed25519;
  pub(crate) mod ssh_rsa;
  pub(crate) mod x25519;
  -const V1_MAGIC: &[u8] = b"age-encryption.org/v1";
  +const AGE_MAGIC: &[u8] = b"age-encryption.org/";
  +const V1_MAGIC: &[u8] = b"v1";
  const RECIPIENT_TAG: &[u8] = b"-> ";
  const MAC_TAG: &[u8] = b"---";
  @@ -136,7 +137,7 @@ mod read {
  pub(super) fn header(input: &[u8]) -> IResult<&[u8], Header> {
  preceded(
  - pair(tag(V1_MAGIC), newline),
  + pair(pair(tag(AGE_MAGIC), tag(V1_MAGIC)), newline),
  map(
  pair(
  terminated(separated_nonempty_list(newline, recipient_line), newline),
  @@ -178,6 +179,7 @@ mod write {
  pub(super) fn header_minus_mac<'a, W: 'a + Write>(h: &'a Header) -> impl SerializeFn<W> + 'a {
  tuple((
  + slice(AGE_MAGIC),
  slice(V1_MAGIC),
  string("\n"),
  separated_list(
  