Rework Fx constructors

[aballano]

Use an object to allow default suspend fx constructor and better discovery for eager one

Usage at the end is like the following:

import arrow.core.computations.either

fun non_suspend() {
  either.eager<Int, Int> { 1 } 
}

suspend fun suspend() {
  either<Int, Int> { 1 } 
}

(Re)fixes #133

[nomisRev]

Single shot computation blocks V1 landing in Arrow Core 🎉 🎉 🎉

[nomisRev]

@JorgeCastilloPrz @aballano merge is not longer blocked, correct?

[JorgeCastilloPrz]

Correct @nomisRev feel free to merge anything you need to. I might need to give the course again pretty soon but until then we can keep moving.