Add constants

[effectfully]

We have some important field constants (see Jubjub), but no way to reference them from within an Expr. They can be embedded directly into an Expr, but it's better to use names for named things.

But maybe it's more important for expression to stay self-contained. Not sure yet.

[kwxm]

About the remark about comparing the characteristic of the field with the number r in Jubjub,hs.
I'm sure you know this, but you can do something like

type P = 52435875175126190479447740508185965837690552500527637822603658699938581184513
type F = GF.Prime P
...
r == GHC.TypeLits.natVal (Proxy:: Proxy P)

I'd been wondering about this anyway because it would be good to check that the big number really is prime.

[Later] It's probably better to use char from Data.Galois.Field.

r == (toInteger $ char (zer::TinyLang.Field.Jubjub.F))

char just uses natVal anyway. It's a bit annoying that we have to compare a Natural with an Integer though.

[jakzale]

So more like a built-in.