chore: move executable code to src/

src/ now contains all source code - in subdirectories, according to the
.cabal component they belong to. This will allow us to put vendored
libraries in the same place - and later split our own code into multiple
components/libraries as well.

src/executable/Main.hs

@@ -0,0 +1,21 @@
+module Main (main) where
+
+import System.IO (BufferMode (..), hSetBuffering)
+
+import qualified PostgREST.CLI as CLI
+
+import Protolude
+
+main :: IO ()
+main = do
+  setBuffering
+  opts <- CLI.readCLIShowHelp
+  CLI.main opts
+
+setBuffering :: IO ()
+setBuffering = do
+  -- LineBuffering: the entire output buffer is flushed whenever a newline is
+  -- output, the buffer overflows, a hFlush is issued or the handle is closed
+  hSetBuffering stdout LineBuffering
+  hSetBuffering stdin LineBuffering
+  hSetBuffering stderr LineBuffering