Improve `doubleDec/floatDec`

[winterland1989]

As the author of mysql-haskell, i'm keen on formatting numeric values into texual values, the comment on doubleDec/floatDec says

-- TODO: Use Bryan O'Sullivan's double-conversion package to speed it up.

But i don't think we really need double-conversion here, since with Builder infrastructure we don't have to estimate texual length and allocate extra memory, we can format them in haskell pretty well IMHO, there's two reference native implementations: blaze-texual and text(Which use a similar Builder type). So is there any interest in merging some related code?

It's not as easy as it seems to be, but i still made some progress on it ; )

[dcoutts]

Sure. What do you have in mind?

[winterland1989]

Happy to see your reply, I'll make a PR soon.

[winterland1989]

Done in #115 ; )

[winterland1989]

I'm considering to add my grisu/girsu_sp to GHC.Float along aside some rules to speed up default show, so that packages rely on it can be improved without modify.

If we go that routine, i think we can add floatDecFast/doubleDecFast builder, which use something like milo to get fast but not shortest dtoa conversion.