Improve parsing performance (#119)

This left-factors the parser for union literals to remove an unnecessary
`try`.  This in turn improves the throughput of `dhall` on the slow
example described in #108 by 56% (2.290 s → 1.465 s)

src/Dhall/Parser.hs

@@ -811,28 +811,22 @@ alternativeType embedded = do
    return (a, b)

unionLit :: Show a => Parser a -> Parser (Expr Src a)
unionLit embedded =
        try unionLit0
    <|>     unionLit1
  where
    unionLit0 = do
        symbol "<"
        a <- label
        symbol "="
        b <- exprA embedded
        symbol ">"
        return (UnionLit a b Data.Map.empty)

    unionLit1 = do
        symbol "<"
        a <- label
        symbol "="
        b <- exprA embedded
        symbol "|"
        c <- alternativeTypes embedded
        d <- toMap c
        symbol ">"
        return (UnionLit a b d)
unionLit embedded = do
    symbol "<"
    a <- label
    symbol "="
    b <- exprA embedded
    let unionLit0 = do
            symbol ">"
            return (UnionLit a b Data.Map.empty)

    let unionLit1 = do
            symbol "|"
            c <- alternativeTypes embedded
            d <- toMap c
            symbol ">"
            return (UnionLit a b d)
    unionLit0 <|> unionLit1

listLit :: Show a => Parser a -> Parser (Expr Src a)
listLit embedded = do