Move containers to detail/container/
Add well-defined formal sequence tools.

Includes concepts, traits, and utilities to decouple the range
manipulation code. There are some extra generic bits that may be moved
to wheels in the future.
Clean up
Fix comment
Rename concepts
Rename forward_as_sequence -> forward_sequence
Add result_of::encode/decode
Reorganize sequence traits

Mostly just naming. `is_sequence` now properly tests all sequences; the
old `is_sequence` is now `is_simple_sequence`.
Add is_sequence_of
Remove empty file
Add forward_sequence support for ICU
Minor changes
Add sequence_iterator
Add docs and reorganize
Rename namespace sequence -> namespace seq
Add is_decayed trait
Made unicode_error inherit virtual and use UTF-8
Add Boost.Exception support
New error handling interface
encode/decode no longer static