Fixed issue where package doesn't have enough dependencies by adding a lens dependency

[mstksg]

The package in its current state suffers from a problem of not requiring any dependencies. (HEAD currently only depends on base, which comes with GHC). This PR adds a lens dependency and rewrites some functionality using lens-based optics, which should be enough to ensure that requiring this package pulls in an obscene amount of transitive dependencies without fail.

[mstksg]

didn't even see #3 before making this; hopefully this is a good step towards resolving it.

[mstksg]

@stettberger added positive_integer_prism, and re-implemented is-positive-integer and is-not-positive-integer in terms of it.

[stettberger]

stettberger@faui48f:/tmp/ispositive$ cabal configure
Warning: The package list for 'hackage.haskell.org' does not exist. Run 'cabal
update' to download it.
Resolving dependencies...
Configuring ispositive-0.2...
stettberger@faui48f:/tmp/ispositive$ cabal build
Building ispositive-0.2...
Preprocessing library ispositive-0.2...
[1 of 1] Compiling Integer.IsPositive ( src/Integer/IsPositive.hs, dist/build/Integer/IsPositive.o )

src/Integer/IsPositive.hs:11:49:
Not in scope: pure' Perhaps you meantpre' (imported from Control.Lens)

Sorry, cannot build this (used libghc-lens-dev from Debian Unstable)