Use the set version of infer

[dghosef]

the current one is way too slow. Also to speed up:

Give each type an int
Use union-find in unify
Turn on optimizations
Go back to the mutable version
Allow the program to be run in the mean time
There are times in which simplifyig the type would be helpful. Take advantage of those
Pass around a loc with a type
Watch the output of unify on a simple program like:

list: {
  Data[#None{}]
  Prepend[value|
    {self Data[#Some {V[value] P[`Prepend Data.]}]}
  ]
}
list Prepend {}. Prepend {}. Prepend {}.