der_derive: rewrite EncodeValue::value_len as simple addition

[dishmaker]

Fixes#1800

This PR replaces

[..].into_iter().try_fold(..)

with a simple addition.

cargo llvm-lines
Before: 57722
After: 56603

./formats$ cargo llvm-lines --release -p x509-cert | head -20
Lines Copies Function name
----- ------ -------------
56603 1235 (TOTAL)
2065 (3.6%, 3.6%) 59 (4.8%, 4.8%) <T as der::encode::Encode>::encoded_len
1791 (3.2%, 6.8%) 25 (2.0%, 6.8%) core::iter::traits::iterator::Iterator::try_fold
1728 (3.1%, 9.9%) 12 (1.0%, 7.8%) der::ord::iter_cmp
1708 (3.0%, 12.9%) 12 (1.0%, 8.7%) <der::reader::slice::SliceReader as der::reader::Reader>::read_nested
1597 (2.8%, 15.7%) 13 (1.1%, 9.8%) <T as der::decode::Decode>::decode
1343 (2.4%, 18.1%) 14 (1.1%, 10.9%) <der::reader::slice::SliceReader as der::reader::Reader>::finish
1240 (2.2%, 20.3%) 20 (1.6%, 12.6%) <T as der::ord::DerOrd>::der_cmp
1176 (2.1%, 22.3%) 21 (1.7%, 14.3%) der::encode::EncodeValue::header
1155 (2.0%, 24.4%) 1 (0.1%, 14.3%) x509_cert::attr::AttributeTypeAndValue::from_delimited_str
995 (1.8%, 26.1%) 1 (0.1%, 14.4%) <x509_cert::attr::AttributeTypeAndValue as core::fmt::Display>::fmt
852 (1.5%, 27.6%) 12 (1.0%, 15.4%) <der::asn1::context_specific::ContextSpecific<T> as der::tag::Tagged>::tag
837 (1.5%, 29.1%) 3 (0.2%, 15.6%) der::encode::Encode::encode_to_vec
754 (1.3%, 30.5%) 1 (0.1%, 15.7%) <x509_cert::ext::pkix::crl::dp::IssuingDistributionPoint as der::encode::EncodeValue>::value_len
664 (1.2%, 31.6%) 8 (0.6%, 16.4%) <T as der::encode::Encode>::encode
624 (1.1%, 32.7%) 12 (1.0%, 17.3%) <core::iter::adapters::zip::Zip<A,B> as core::iter::adapters::zip::ZipImpl<A,B>>::next
570 (1.0%, 33.7%) 15 (1.2%, 18.5%) <core::slice::iter::Iter<T> as core::iter::traits::iterator::Iterator>::next
564 (1.0%, 34.7%) 12 (1.0%, 19.5%) <core::iter::adapters::zip::Zip<A,B> as core::iter::adapters::zip::ZipImpl<A,B>>::new