initial import of icu string serializer

[ct-clmsn]

serialization functions to support direct icu::UnicodeString interactions.

[hkaiser]

Chris, thanks for contributing this code. I'm not sure however if we should add it to HPX as we don't have any dependencies to ICU in any way. This looks like more of something you'd want to keep local to your application.

But I'm open to discussing this. What do others think?

[ct-clmsn]

Understood; figured if icu was a boost::regex dependency, it might not be a particularly big deal but, I can understand the concern about additional dependencies.

[hkaiser]

Understood; figured if icu was a boost::regex dependency, it might not be a particularly big deal but, I can understand the concern about additional dependencies.

Yah, it's an optional Boost.Regex dependency. We could add it as an optional dependency (make it available if Boost.Regex has it enabled), but we're planning to move to std.regex at some point which would make this unnessecary.