Cosmetic and documentation improvements for abc::_raw_vextr_impl_base and related classes

Make the name of a few methods shorter, and document several methods in
abc::_raw_vextr_impl_base-related classes.
Merge remote-tracking branch 'remotes/origin/master' into vs2010
Fix string comparison template arguments

Remove unnecessary template argument default from str_base_::compare_to().

Remove unused (copy & paste error?) template argument to str_base_’s
relational operator.

Add test case to make sure NUL-termination does not affect
str_base_::compare_to().
Fix ill-conceived implementation of ABC_CLASS_PREVENT_COPYING for MSC16

Only declare the methods to be made private, don’t actually define them.
Merge remote-tracking branch 'remotes/origin/master' into vs2010