accurate IteratorRandom::choose()

f64 reciprocal is inexact, with residuals on the order of 1 / 2^54.

For example: the probability gen_bool(1.0 / 3) == true is:
6004799503160661/18014398509481984
Using gen_range is exact for all values of `consumed`.

NOTE: this is a value stability-breaking change

fixes #1058

Author: kazcw

src/seq/mod.rs

@@ -324,8 +324,7 @@ pub trait IteratorRandom: Iterator + Sized {
                     return result;
                 }
                 consumed += 1;
-                let denom = consumed as f64; // accurate to 2^53 elements
-                if rng.gen_bool(1.0 / denom) {
+                if gen_index(rng, consumed) == 0 {
                     result = elem;
                 }
             }