Derive `Bounded` Instance for `TokenType`

[prikhi]

In one of my projects I wanted to convert my molokai vim colorscheme into a skylighting Style.

To build the tokenStyles field, I wanted to map over a list of all TokenTypes & pass them through to a function that uses case ... of:

    tokenStyles :: M.Map TokenType TokenStyle
    tokenStyles =
        M.fromList $ [(tok, tokenStyle tok) | tok <- allTokens]
    tokenStyle :: TokenType -> TokenStyle
    tokenStyle = \case
        KeywordTok ->
            bold $ color "#F92672"
        DataTypeTok ->
            color "#66D9EF"

I can't define allTokens generically, I'm stuck using discrete values for the start & end of the range:

    tokenStyles :: M.Map TokenType TokenStyle
    tokenStyles =
        M.fromList $ [(t, tokenStyle t) | SkyToken t <- [minBound .. maxBound]]

-- | Custom type for deriving "Bounded TokenType".
newtype SkyToken = SkyToken TokenType
    deriving newtype (Enum)

instance Bounded SkyToken where
    minBound = SkyToken KeywordTok
    maxBound = SkyToken NormalTok

With a derived Bounded instance for TokenType, I can drop my SkyToken newtype & ensure any newly added values to the TokenType type get pulled into my list of "all tokens".

---

Add Bounded to the derived instances for the TokenType type. This allows consumers to use [minBound .. maxBound] to generate a list of all token types when writing a Style.