Before Boost 1.64, there was no test logging function
for std::nullptr_t.
Add a logging struct to Boost to deal with this, and some
macros to assist in similar cases. These macros are bit
untidy-looking, but due to GCC bug #56480, we can't use
namespace aliasing to solve this. From Boost 1.64 onwards,
these namespaces are not needed at all.
Remove some code added to work around lack of nullptr printing
in the past.
These macros, and the nullptr printer, can be removed when
the Boost min version is 1.64 or greater.
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