Improve documentation: copy over from Parsley

[david-davies]

Closes #39

I've copied over the docs from parsley.
I've tried to make sure to adapt it to Haskell properly.
I haven't copied over examples because my HLS was getting upset if I tried to run examples in comments.

Some modules used to re-export the entire module; I have documented using explicit exports to make it look nicer, but also re-exported the module to avoid breaking API -- this will produce warnings for duplicate exports.

[david-davies]

Fails due to a parsing error in GHC 8.10 only.
Error message says this is due to the curly brace,

gigaparsec/src/Text/Gigaparsec/Internal/Token/Lexer.hs

Lines 141 to 147 in 332d364

	data Lexeme =
	-- | The parsers do consume whitespace
	Lexeme
	-- | This turns a non-lexeme parser into a lexeme one by ensuring whitespace is consumed after the parser.
	{ apply :: !(forall a. Parsec a -> Parsec a) -- this is tricky...
	-- | Parse the given string.
	, sym :: !(String -> Parsec ())

I'm assuming it's upset at the newline before the brace, and that:

data Lexeme =  
   -- | The parsers do consume whitespace 
   Lexeme {
       -- | This turns a non-lexeme parser into a lexeme one by ensuring whitespace is consumed after the parser. 
         apply :: !(forall a. Parsec a -> Parsec a) -- this is tricky... 
       -- | Parse the given string. 
       , sym :: !(String -> Parsec ())

would fix this?