cast.hpp was moved to the numeric_cast library
Add additional tests.
Remove unnecessary includes from polymorphic_cast.hpp.
Remove unnecessary includes from implicit_cast_fail.cpp.
Remove unnecessary includes from cast_test.cpp.
Remove use of mpl::identity from implicit_cast.hpp.
Merge pull request #4 from boostorg/pull-request/cast-prune-dependencies

Prune conversion dependencies
Drop dependency to the boost_unit_test_framework library in tests
remove numeric_cast_test that is a copy of numeric/conversion/test/numeric_cast_test.cpp
Merge branch 'develop'