Implement packed fields

[tristantarrant]

Quoting the protobuf docs:

repeated fields of a primitive type (any scalar type that is not string or bytes) can be declared as “packed”. In proto2 this is done using the field option [packed=true]. In proto3 it is the default.

Instead of being encoded as one record per entry, they are encoded as a single LEN record that contains each element concatenated. To decode, elements are decoded from the LEN record one by one until the payload is exhausted. The start of the next element is determined by the length of the previous, which itself depends on the type of the field.