Generalized enc_vector (caution: ABI has chanced)

It is now possible to specify the bitvector type used to store
the encoded deltas.
Example: `enc_vector<coder::elias_delta, 32, rrr_vector<63>>`
uses Elias-delta code, samples every 32th value, and
uses` rrr_vector<63>` to store the string of encoded deltas.

Default bitvector type is `bit_vector`.