Approximate FSE encoding costs for selection

Approximate FSE encoding costs for selection

Estimate the cost for using FSE modes `set_basic`, `set_compressed`, and
`set_repeat`, and select the one with the lowest cost.

* The cost of `set_basic` is computed using the cross-entropy cost
  function `ZSTD_crossEntropyCost()`, using the normalized default count
  and the count.
* The cost of `set_repeat` is computed using `FSE_bitCost()`. We check the
  previous table to see if it is able to represent the distribution.
* The cost of `set_compressed` is computed with the entropy cost function
  `ZSTD_entropyCost()`, together with the cost of writing the normalized
  count `ZSTD_NCountCost()`.