New functions to implement:

- mpfr_swap, mpfr_mul_mpz, mpfr_div_mpz

Miscellaneous:

- implement a C++ wrapper [see with Torbjo"rn and/or Fabrice]

- deal correctly with all special values (zero, infinities, NaN)

- for all instructions that call malloc/realloc/etc, handle error cases
  (for example return an int indicating if an error occurred)

- in all functions that do rounding, return an int indicating if the result
  is exact (i.e. no bit was lost) or not (like in mpfr_div_ui)

- protect macros:
  #define SET_ZERO(x)  ---> #define MPFR_SET_ZERO(x)