[boost] Boost.Strings

The fixed_string is already in the review schedule and a proposal for immutable strings was published recently. Wouldn't it be good to combine these (and also maybe the flex string) into a boost.strings library. That way it would be easier to ensure that all string types work properly together. It could also be possible to do some optimizations in the interfaces since each string class can assume the others are available e.g. a fixed_string constructor in the immutable string.