guard hexdigest() doctest by cfg(feature=std)

[ignatenkobrain]

Fixes: #34

[mtn]

Hey @ignatenkobrain!

I ran into this as well (using hexdigest as noted int he documentation, being unable to compile). I had changed to .digest(). Do you mind explaining how what you're doing works, and if I should be doing something similar? Thanks! 🎈

[ignatenkobrain]

@mtn the hexdigest() method is available only when feature std is enabled (you can see it here).

So if you have same error, you probably have default-features = false or smth like that.

[mitsuhiko]

Will be done with the rename to sha1-smol.