requestBodyLength :: Maybe Word64

I think the choice of Word64 makes sense without further explanation. The perhaps surprising part is the use of `Maybe`. The reason for this is that in the case of a chunked request body, there's no way to know the size of the request body from the headers alone. So the logic for populating this field would go something like:

• If content-length is supplied by the user, parse it and populate the field with Just content-length.
• If the request is chunked, populate with Nothing.
• Otherwise, there is a zero-sized request body, and populate with Just 0.