The UT_ prefix was primarily for "internal" macros. This commit seeks
to more consistently "namespace" UnitTest++ macros with the UNITTEST_
prefix, while also marking these particular macros as "private"
implementation details with the IMPL_ wart.
UNITTEST_ now prefixes the implementation of all CHECK macros.
Additionally, the build option UNITTEST_ENABLE_SHORT_MACROS can be
shut off, disabling the creation of the existing short forms. This is
helpful for users who may have conflicts with their projects or other
libs.