PER: extension groups with one field

The current PER encoder for sequences/sets treats extension groups with one field as if they were single extensions.

Example:

Seq ::= SEQUENCE {
  ...,
  f1 BIT STRING (SIZE (1..8)) OPTIONAL,
  [[ f2 BIT STRING (SIZE (1..8)) OPTIONAL ]]
}

In this example f1 needs to be encoded as if it was a field in the sequence, while f2 needs to be encoded as if it was a sequence with one field (including the optional bit). Currently both f1 and f2 are encoded as if they were fields.

/cc @aknappqwt @mmagyari