Expose currently hidden isWhitespace from xml-conduit

[tolysz]

This function is useful outside the module itself. It would be beneficial to expose it.

[k0ral]

One of the builds fails, for some unrelated reason. I'll have a look at it.

[k0ral]

CI build should be fixed now, but your change must be rebased first.

Also, now that I think about it, I am not in favor of exporting isWhitespace as is, for the following reasons:

• the Text.XML.Stream.Parse module only exports conduits so that you never have to deal with individual Events ; isWhitespace lies in a different, lower level of abstraction, and hardly has a legitimate place in the API of this module;
• a better place could be in a new Text.XML.Stream.Parse.Internal module, which would export low-level implementation-detail functions (after all, why just isWhitespace ?); but that still looks like a glorified encapsulation breakage;
• the name isWhitespace sounds like a misnomer, considering its implementation; e.g. does it really make sense for EventBeginDoctype to be called a whitespace ? If we are to export this function, then I would prefer we gave it a better name first.

IMHO, this is one of the very few cases where duplicating the function in your project is a better tradeoff.