der: rewrite der_ord as match without try! ? operator

[dishmaker]

Fixes #1798

Should produce faster compile times for anything using der.

x509-cert with new der_ord:

./formats$ CARGO_PROFILE_RELEASE_LTO=fat cargo llvm-lines --release -p x509-cert | head -20
  Lines                Copies              Function name
  -----                ------              -------------
  58324                1305                (TOTAL)
   2241 (3.8%,  3.8%)    12 (0.9%,  0.9%)  <der::reader::slice::SliceReader as der::reader::Reader>::read_nested
   1826 (3.1%,  7.0%)    24 (1.8%,  2.8%)  <core::ops::index_range::IndexRange as core::iter::traits::iterator::Iterator>::try_fold
   1711 (2.9%,  9.9%)    59 (4.5%,  7.3%)  <T as der::encode::Encode>::encoded_len
   1668 (2.9%, 12.8%)    12 (0.9%,  8.2%)  der::ord::iter_cmp
   1663 (2.9%, 15.6%)    23 (1.8%, 10.0%)  core::iter::traits::iterator::Iterator::try_fold
   1597 (2.7%, 18.4%)    13 (1.0%, 11.0%)  <T as der::decode::Decode>::decode
   1343 (2.3%, 20.7%)    14 (1.1%, 12.0%)  <der::reader::slice::SliceReader as der::reader::Reader>::finish
   1176 (2.0%, 22.7%)    21 (1.6%, 13.6%)  der::encode::EncodeValue::header
   1167 (2.0%, 24.7%)     1 (0.1%, 13.7%)  x509_cert::attr::AttributeTypeAndValue::from_delimited_str
   1160 (2.0%, 26.7%)    20 (1.5%, 15.2%)  <T as der::ord::DerOrd>::der_cmp
    891 (1.5%, 28.2%)     1 (0.1%, 15.3%)  <x509_cert::attr::AttributeTypeAndValue as core::fmt::Display>::fmt
    852 (1.5%, 29.7%)    12 (0.9%, 16.2%)  <der::asn1::context_specific::ContextSpecific<T> as der::tag::Tagged>::tag
    843 (1.4%, 31.1%)     3 (0.2%, 16.5%)  der::encode::Encode::encode_to_vec
    675 (1.2%, 32.3%)    15 (1.1%, 17.6%)  <core::slice::iter::Iter<T> as core::iter::traits::iterator::Iterator>::next
    664 (1.1%, 33.4%)     8 (0.6%, 18.2%)  <T as der::encode::Encode>::encode
    624 (1.1%, 34.5%)    12 (0.9%, 19.2%)  <core::iter::adapters::zip::Zip<A,B> as core::iter::adapters::zip::ZipImpl<A,B>>::next
    564 (1.0%, 35.4%)    12 (0.9%, 20.1%)  <core::iter::adapters::zip::Zip<A,B> as core::iter::adapters::zip::ZipImpl<A,B>>::new

[tarcieri]

@dishmaker retroactive thought: you could maybe try adding an #[inline] annotation and see if that improves inlining