Add some dynamic_size related traits and other infrastructure.
Initial string_view commit using placeholder implementation in lieu of an actual N3585 implementation. Include dynamic size support.
Add tentative support for types like "const char*" where the data essentially knows its own length.
Unstable work-in-progress; fix cut-and-paste bug in insert().
Preliminaries for implementing, testing, more operational functions.
Merge branch 'feature/2layer-dynamic' into feature/2layer

Conflicts:
	include/boost/btree/index.hpp
Fix merge conflict duplicate line.