Due to BER: - using `ber` features makes DER reader slow (about 20% slower) - `read_nested` now includes primitive `DecodeValue`s too (added in #1877) (partial fix in #1916) - ~indefinite flag in `Length` (#1894)~ - ~users of pure DER need to compile BER code too (maybe add feature flag?) (added in #1899)~ - ~constructed flag in `Header` (will be added in #1931)~