Speed up Lexer.signed

`L.signed L.decimal` now has minimal overhead over L.decimal on it's
own. We did sacrifice the possibility to write a (redundant) `+` sign
or white space between the minus and the number.

src/MA/Parser/Lexer.hs

@@ -94,5 +94,5 @@ float = lexeme L.float
 {-# INLINE float #-}
 
 signed :: (MonadParser m, Num a) => m a -> m a
-signed = L.signed spaceConsumer
+signed p = option id (char '-' *> return negate) <*> p
 {-# INLINE signed #-}