der: Tracking issue for deoptimiztions since `v0.7`

[dishmaker]

Due to BER:

• read_nested now includes primitive DecodeValues too
  (added in der: add Reader::read_value, auto-nest DecodeValue #1877)
  (partial fix in der: auto read_nested in trait: DecodeValue::decode_value_nested #1916)
• indefinite flag in Length
  (der: add indefinite flag to Length #1894)
• users of pure DER need to compile BER code too (maybe add feature flag?)
  (added in der: constructed OctetString support #1899)
• constructed flag in Header
  (will be added in Negative test case for constructed, definite-length octet string #1931)

[tarcieri]

Without data to show these are "deoptimizations" this is just a list of things that changed