Correct parenthesisation in generic Show

[tomjaguarpaw]

This PR fixes a number of issues with the gshow derived in this module

• Show operators correctly, rather than putting them in quotation marks
• Don't show spurious I
• Parenthesise properly (and thus derive gshowsPrec instead of gshow directly)

I appreciate that this package is for "simple basic examples" rather than "fully working but less simple code" but I would question the value of the simple examples if they're just completely wrong for the stated purpose. I think this PR isn't too much more complex than what went before and I think it would be useful to have a real example of Generics.SOP that people can drop in to code that's expected to work.

Anyway, if you really don't want to incorporate this, no hard feelings. I can always create a new package.

Data types

data T = A (Maybe ()) Bool | B Bool

infix 7 :**
data O1 a b = a :** b

infix 6 :++
data O2 a b = a :++ b

data R = MkR1 { r1 :: Maybe (), r2 :: Bool } | MkR2 Bool

Before

A Just () True
I (Just ()) ":**" I (Just ())
I I (Just ()) ":++" I (Just ()) ":**" I I (Just ()) ":++" I (Just ())
I I (Just ()) ":**" I (Just ()) ":++" I I (Just ()) ":**" I (Just ())
MkR1 {r1 = Just (), r2 = True}
Just A Just () True
Just I (Just ()) ":**" I (Just ())
Just MkR1 {r1 = Just (), r2 = True}

After

A (Just ()) True
Just () :** Just ()
(Just () :++ Just ()) :** (Just () :++ Just ())
Just () :** Just () :++ Just () :** Just ()
MkR1 {r1 = Just (), r2 = True}
Just (A (Just ()) True)
Just (Just () :** Just ())
Just MkR1 {r1 = Just (), r2 = True}

[edsko]

Rebased and merged in #12 , and released as https://hackage.haskell.org/package/basic-sop-0.3.0 . Thanks so much!