Add const_container
beast.streams improvements:
* New basic_std_ostream wraps any std::stream.
* New debug_ostream redirects output to debugger if attached.
New unit_test framework:
* Header-only!
* No external dependencies or other beast modules
* Compilation options allow for:
  - Stand-alone application to run a single test suite
  - Stand-alone application to run a set of test suites
  - Global suite of tests inline with the host application
  - Disable test suite generation completely