ecdsa: adjust for upcoming der API changes

Make ecdsa crate compatible with upcoming changes to `Header` struct in
the `der` crate.

Author: dwhjames

Cargo.lock

@@ -15,6 +15,7 @@ members = [
 opt-level = 2
 
 [patch.crates-io]
+der = { git = "https://github.com/RustCrypto/formats" }
 elliptic-curve = { git = "https://github.com/RustCrypto/traits" }
 
 # A global patch crates-io block is used to avoid duplicate dependencies

Cargo.toml

@@ -204,10 +204,10 @@ where
 
     fn decode<R: Reader<'a>>(reader: &mut R) -> der::Result<Self> {
         let header = Header::peek(reader)?;
-        header.tag.assert_eq(Tag::Sequence)?;
+        header.tag().assert_eq(Tag::Sequence)?;
 
         let mut buf = SignatureBytes::<C>::default();
-        let len = (header.encoded_len()? + header.length)?;
+        let len = (header.encoded_len()? + header.length())?;
         let slice = buf
             .get_mut(..usize::try_from(len)?)
             .ok_or_else(|| reader.error(Tag::Sequence.length_error()))?;