feat: add time bounds for ReductionSystems

[BoltonBailey]

This PR adds a number of features to the ReductionSystem library, mainly around counting the steps that a reduction takes.

Specifically, it includes:

• Definitions for saying a reduction system reduces one term to another in a specific number of steps, or within a bound on the number of steps.
• Defines TerminalReductionSystem as an extension of ReductionSystem which has terminal states.
• Defines a way of constructing a TerminalReductionSystem from a map \sigma -> Option \sigma

These are a prerequisite to defining time bounds, perhaps for multiple models of computation, but in particular for Turing machines, see PR #269.