make ‘hexadecimal’ and ‘octal’ more powerful

Various languages may vary in how hexadecimal and octal literals should
be prefixed. Following the spirit of the new lexer we leave this to
programmer to decide.

Text/Megaparsec/Lexer.hs

@@ -190,18 +190,26 @@ decimal :: Stream s m Char => ParsecT s u m Integer
 decimal = nump "" C.digitChar
 
 -- | Parse an integer in hexadecimal representation. Representation of
--- hexadecimal number is expected to be according to Haskell report, that
--- is, it should be prefixed with “0x” or “0X” prefix.
+-- hexadecimal number is expected to be according to Haskell report except
+-- for the fact that this parser doesn't parse “0x” or “0X” prefix. It is
+-- reponsibility of the programmer to parse correct prefix before parsing
+-- the number itself.
+--
+-- For example you can make it conform to Haskell report like this:
+--
+-- > hexadecimal = char '0' >> char' 'x' >> L.hexadecimal
 
 hexadecimal :: Stream s m Char => ParsecT s u m Integer
-hexadecimal = C.char '0' >> C.char' 'x' >> nump "0x" C.hexDigitChar
+hexadecimal = nump "0x" C.hexDigitChar
 
 -- | Parse an integer in octal representation. Representation of octal
--- number is expected to be according to Haskell report, that is, it should
--- be prefixed with “0o” or “0O” prefix.
+-- number is expected to be according to Haskell report except for the fact
+-- that this parser doesn't parse “0o” or “0O” prefix. It is responsibility
+-- of the programmer to parse correct prefix before parsing the number
+-- itself.
 
 octal :: Stream s m Char => ParsecT s u m Integer
-octal = C.char '0' >> C.char' 'o' >> nump "0o" C.octDigitChar
+octal = nump "0o" C.octDigitChar
 
 -- | @nump prefix p@ parses /one/ or more characters with @p@ parser, then
 -- prepends @prefix@ to returned value and tries to interpret the result as