Remove EarlyGoalError and tuple evaluation

[brandonwillard]

Closes #10.

These changes break all of the old API and effectively make this an entirely different implementation of miniKanren. At the very least, these changes will come with a new major version number.

It also removes the arith module/goals and the prime.py example. Both consisted of marginally useful non-relational goals that should be implemented outside of the kanren package, or—at best—implemented as constraints (e.g. an addo(x, y, z) goal that constructs a constraint to evaluate add(x, y) == z when x, y, and z are ever ground).