Add Equal to move.Move

[artasparks]

We should add an Equal method to move.Move. This allows us to use cmp.Equal

See

• Cmp documentation: https://pkg.go.dev/github.com/google/go-cmp/cmp
• Move: https://github.com/otrego/clamshell/tree/master/core/move

Note that Equal methods must have the following properties

If the values have an Equal method of the form "(T) Equal(T) bool" or "(T) Equal(I) bool" where T is assignable to I, then use the result of x.Equal(y) even if x or y is nil. Otherwise, no such method exists and evaluation proceeds to the next rule.