Use an {encode,decode}_bin-like interface for encoding/decoding

[pascutto]

One remaining spot of allocations in index lives in the interface between index and repr, in the Key.encode: t -> string and Value.encode: t -> string functions of the functor argument.

These function allocate strings, but these are then blit into already existing buffers, then blit again in a file. This allocation should be avoided, by using an interface closer to repr's encode_bin.

Similar reasoning applies to decode, as this is heavily used during merges to decode all data keys.

[mattiasdrp]

See #287