crypto_rand_double() should produce all possible outputs on platforms with 32-bit int

On 32-bit platforms, crypto_rand_double() only produces 1 in every 2 million possible values between 0 and 1.

This happens because:

• crypto_rand_double() divides a random unsigned int by UINT_MAX
• an unsigned int on 32-bit platforms is 32 bits
• the mantissa on a double is 53 bits

So crypto_rand_double() doesn't fill the lower 21 bits with random values.

This makes the rep_hist_format_hs_stats() noise more predictable on 32-bit platforms.

This fix shouldn't affect the unit tests, because they pass on 64-bit.