possible minor improvement (suggestion)

[Matej Kosik]

Hello,

At the moment Bitstring.t is not an abstract type. I do not think this
is ideal.

The programmer now sees more than he needs to see.
Ocaml type-error messages (which could be really tricky) are important
and if we might want to always see "Bitstring.t" instead of the
occasional "string * int * int".

E.g.

Bitstring.t -> Bitstring.t

is easier to understand than:

string * int * int -> string * int * int