der: remove generic <T> from Reader::finish

[dishmaker]

Depends on:

• ecdsa: der: use impl EncodeValue/DecodeValue on SignatureRef signatures#965

Before:

./formats$ cargo llvm-lines -p der --all-features | head -n 10
  Lines                Copies              Function name
  -----                ------              -------------
  68510                1007                (TOTAL)
   6422 (9.4%,  9.4%)    29 (2.9%,  2.9%)  der::asn1::any::AnyRef::decode_as
   3334 (4.9%, 14.2%)    33 (3.3%,  6.2%)  <der::reader::slice::SliceReader as der::reader::Reader>::finish
   2310 (3.4%, 17.6%)    10 (1.0%,  7.1%)  der::asn1::integer::value_cmp

After:

./formats$ cargo llvm-lines -p der --all-features | head -n 10
  Lines                Copies             Function name
  -----                ------             -------------
  65769                975                (TOTAL)
   6879 (10.5%, 10.5%)  29 (3.0%,  3.0%)  der::asn1::any::AnyRef::decode_as
   2310 (3.5%, 14.0%)   10 (1.0%,  4.0%)  der::asn1::integer::value_cmp
   1768 (2.7%, 16.7%)    1 (0.1%,  4.1%)  der::tag::_::<impl arbitrary::Arbitrary for der::tag::Tag>::try_size_hint::{{closure}}