fixed building on 32-bit arch

Release 2.0

CHANGES:

  - Switch to bytes-based implementation. This is simpler and faster
    (up to 30% faster).
    `Bitv_string` is still there but deprecated (now identical to `Bitv`).
  - Functions `to_int...` do not fail anymore when bit vectors are small
    (and behave as if high bits are zero).
  - New function `tanimoto` (Tanimoto score) (contribution by François Berenger)

Distribution 1.6

Distribution 1.5

Distribution 1.4

release 1.3

release 1.2

opam file is generated at configuration time
(so that version number is always correct)