This is the normal standard way in C, C++ as well as Java and there's no
longer any reason for ICU to be different. The various internal macros
providing custom boolean constants can all be deleted and code as well
as documentation can be updated to use lowercase true/false everywhere.
The definition of max_align_t is not guaranteed to be available unless
the appropriate header is included. Since use of <stddef.h> from C++ is
deprecated, that's <cstddef>, and max_align_t is thus defined under the
std namespace rather than in the global namespace.
