Permutation groupoid

The `Permutations` submodule currently provides some basic algorithms for decomposing permutations, which are used when converting wiring diagrams into expressions. We should extend this module to implement the groupoid of finite sets and bijections as an instance of the theory of groupoids (#230), following the general pattern established for `FinSet` and `FinRel`:

- support both `FinBij = Core(FinSet)` and its skeleton, the [permutation groupoid](https://ncatlab.org/nlab/show/permutation+groupoid)
- an abstract type for morphisms in this category
- concrete types for particular computational representations, including at minimum the vector and cycle representations