PER: remove trailing 0 bits from bitstrings

According to the PER standard:

"Where there are no PER-visible constraints and Rec. ITU-T X.680 | ISO/IEC 8824-1, 22.7, applies the value shall be encoded with no trailing 0 bits (note that this means that a value with no 1 bits is always encoded as an empty bit string). Where there is a PER-visible constraint and Rec. ITU-T X.680 | ISO/IEC 8824-1, 22.7, applies (i.e., the bitstring type is defined with a "NamedBitList"), the value shall be encoded with trailing 0 bits added or removed as necessary to ensure that the size of the transmitted value is the smallest size capable of carrying this value and satisfies the effective size constraint."

The current version of the PER encoder does not remove trailing 0 bits in any situation.

/cc @aknappqwt @mmagyari