feat(algebra/quandle): racks and quandles (#4247)

This adds the algebraic structures of racks and quandles, defines a few examples, and provides the universal enveloping group of a rack.

A rack is a set that acts on itself bijectively, and sort of the point is that the action `act : α → (α ≃ α)` satisfies
```
act (x ◃ y) = act x * act y * (act x)⁻¹
```
where `x ◃ y` is the usual rack/quandle notation for `act x y`.  (Note: racks do not use `has_scalar` because it's convenient having `x ◃⁻¹ y` for the inverse action of `x` on `y`.  Plus, associative racks have a trivial action.)

In knot theory, the universal enveloping group of the fundamental quandle is isomorphic to the fundamental group of the knot complement.  For oriented knots up to orientation-reversed mirror image, the fundamental quandle is a complete invariant, unlike the fundamental group, which fails to distinguish non-prime knots with chiral summands.

docs/references.bib

@@ -397,3 +397,30 @@ @book{cassels1967algebraic
   year={1967},
   publisher={Academic Pr}
 }
+
+@Article{Joyce1982,
+  author    = {David Joyce},
+  title     = {A classifying invariant of knots, the knot quandle},
+  journal   = {Journal of Pure and Applied Algebra},
+  year      = {1982},
+  volume    = {23},
+  number    = {1},
+  month     = {1},
+  pages     = {37--65},
+  doi       = {10.1016/0022-4049(82)90077-9},
+  publisher = {Elsevier {BV}}
+}
+
+@Article{FennRourke1992,
+  author    = {Fenn, Roger and Rourke, Colin},
+  journal   = {Journal of Knot Theory and its Ramifications},
+  title     = {Racks and links in codimension two},
+  year      = {1992},
+  issn      = {0218-2165},
+  number    = {4},
+  pages     = {343--406},
+  volume    = {1},
+  doi       = {10.1142/S0218216592000203},
+  keywords  = {57M25 (57N10)},
+  mrnumber  = {1194995}
+}