Need an `Integer -> Maybe Positive` function

[blamario]

https://github.com/Liqwid-Labs/plutus-extra/blob/master/plutus-numeric/src/PlutusTx/Positive.hs doesn't export any way to construct a Positive value except from a literal, using the quasiquoter. I'm currently using a ridiculous workaround of

toPositive :: Integer -> Positive
toPositive = unsafeFromBuiltinData . toBuiltinData

(in this case I ensure the argument is positive beforehand).

Alternatively, I could use

nonZero :: Natural -> Maybe Positive

[kozross]

I feel having conversions out of both Integer and Natural makes some sense.