der: SetOfVec::insert can break the structure on error

[roblabla]

The way SetOfVec::insert is implemented results in the internal structure containing an unsorted duplicate value, because it first inserts the value, and then check whether it's a dupe while simultaneously sorting it:

• SetOfVec::insert: https://github.com/RustCrypto/formats/blob/master/der/src/asn1/set_of.rs#L268-L271
• der_sort: https://github.com/RustCrypto/formats/blob/master/der/src/asn1/set_of.rs#L456-L473

Could der_sort perhaps remove the duplicate so the structure is still valid in case the insert function returns an error?