Add --quiet option for dhall decode

[sjakobi]

This is intended primarily for investigating decoding performance.

[sjakobi]

Sample measurement from my machine:

$ echo 'https://raw.githubusercontent.com/vmchale/cpkg/0003ec5af2859008424aec06c2db0437a6a22ff9/pkgs/pkg-set.dhall' | dhall --alpha | dhall encode > tmp
$ bench "dhall decode --quiet --file tmp"
benchmarking dhall decode --quiet --file tmp
time                 415.9 ms   (409.3 ms .. 430.8 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 417.1 ms   (414.8 ms .. 418.8 ms)
std dev              2.680 ms   (0.0 s .. 3.029 ms)
variance introduced by outliers: 19% (moderately inflated)
$ wc -c tmp
5147490 tmp
$ ghc -e '5147490 / (1024 * 1024) / 0.4159'
11.803387850343851

So 11.8 MB/s.

[german1608]

(style comment) use unless (from Control.Monad):

Suggested change

	if quiet
	then return ()
	else do
	let doc =
	Dhall.Pretty.prettyCharacterSet
	characterSet
	(Dhall.Core.renote expression :: Expr Src Import)
	unless quiet $ do
	let doc =
	Dhall.Pretty.prettyCharacterSet
	characterSet
	(Dhall.Core.renote expression :: Expr Src Import)

[sjakobi]

Yeah. When I started contributing to dhall, I often wanted to use unless and similar utils too, but realized that Gabriel mostly sticks to if. These days, I find this if-style actually more readable than the implicit negation of unless.