Before this commit we were using overloading for concrete structures and
type classes for abstract ones.
This is the first of series of commits that implement this modification
Many theorems for division rings and fields have stronger versions for discrete fields, where we
assume x / 0 = 0. Before, we used primes to distinguish the versions, but that has the downside
that e.g. for rat and real, all the theorems are equally present. Now, I qualified the weaker ones
with division_ring.foo or field.foo. Maybe that is not ideal, but let's try it.
I also set implicit arguments with the following convention: an argument to a theorem should be
explicit unless it can be inferred from the other arguments and hypotheses.
I changed the definition of pow so that a^(succ n) reduces to a * a^n rather than a^n * a.
This has the nice effect that on nat and int, where multiplication is defined by recursion on the right,
a^1 reduces to a, and a^2 reduces to a * a.
The change was a pain in the neck, and in retrospect maybe not worth it, but oh, well.
