Align with main: two ways of parsing repeated packable fields.

It is a requirement for parsing code to handle packed and unpacked
forms on the wire for repeated packable fields. This change aligns
the javanano's behavior with the java's.

Bonus: optimize array length calculation when parsing repeated
fixed-size-element-type fields.

Bonus 2: lose "xMemoizedSerializedSize" for repeated enum fields,
and make the serialized size calculation match that for repeated
int32 fields.

Change-Id: I8a06103d9290234adb46b0971b5ed155544fe86a