Add exponentiation for Integers

[thomasdziedzic]

I would like to write something like:
2^64 and get back 18446744073709551616 like in haskell

I know about the Nat version;
Nats have a power function defined under Data.Nat but this is slow for power 2 64

@ohad mentioned I should link this issue to #29 as it might be easy to add this feature along with the Double constants.

[mr-infty]

I've made a draft PR (#1084) that fixes this issue. I've chosen to ignore issue #29, as it seems unrelated. Incidentally, I'm still working on fixing the latter issue, but this will take a bit more time.

[gallais]

Fixed by #1063 (cf. Data.Monoid.Exponentiation)