Example and NaN fix

[chrysn]

This PR does 3 things in commits that should be independent enough that you can cherry-pick if you don't like the bundle:

• Adds an example for easy cargo run --example osrng that tests the OS's RNG and prints concrete outputs (and gives concrete example values to put into the block size).
• Switches around the comparisons to make sure NaNs are errors. Closes: nist_freq_block does not err when returning NaN #1
• Replaces the ad-hoc igamma implementation with one from the special crate.

Big caveat that I only just found while doing further testing: Apparently the special crte does require alloc; checking to see if that can be made optional.

[chrysn]

PR'd on the other end as stainless-steel/special#13.

[ryankurte]

lgtm (though i created a conflict by merging #3, soz)

PR'd on the other end as stainless-steel/special#13.

nice! no reason to block i think / can update when it's available

[chrysn]

The conflict is resolved by merging the main branch into here. An extra change updates the special dependency, because upstream merged and released the no_alloc fix.