Provide bare and opt set operations

[josevalim]

This pull request breaks set operations (union, intersections, differences, and negations) into bare and opt (optimized). This is a large PR but most changes are mechanical:

• All functions outside of the Descr module use the optimized versions
• The Descr module uses the bare versions for everything, except for to_quoted and type "mutators" (tuple_update, map_delete, etc)

Therefore, to review this code, it is best to open up the descr.ex module and see if the usage of opt_* functions within that module are acceptable. There are not many of them.