Pick one from weighted list testing and codestyle

[webmaster128]

Some imporovements on top of https://github.com/noislabs/nois/pull/29

• Test non-Cloneable elements for pick and non-Copyable elements for pick_one_from_weighted_list
• Accept slice instead of vector only
• Refactor 2 loops into 1 loop
• Don't expose variable name total_weight in error message

[kaisbaccour]

lgtm