bench: Add benchmark for hex parser

bench/Copar/Parser/BenchLexer.hs

@@ -16,6 +16,7 @@ benchmarks = bgroup "Copar.Parser.Lexer"
   [ benchColon
   , benchDecimal
   , benchFloat
+  , benchHex
   , benchName
   , benchWhitespace
   ]
@@ -40,6 +41,12 @@ benchFloat = bgroup "float"
   , benchp @Double "signed" (L.signed L.float) "2.87182364"
   ]
 
+benchHex :: Benchmark
+benchHex = bgroup "hex"
+  [ benchp @Int "single" L.hex "0x1"
+  , benchp @Int "many" L.hex "0x123456789ABCDEF"
+  ]
+
 benchName :: Benchmark
 benchName = bgroup "name"
   [ benchp "one letter" L.name "a"