The user-supplied constant names are used to declare an internal enum
type. They end up sharing a namespace with some internal library
values. Those internal values had names not prefixed with underscores.
As a result, it was impossible to declare enum constants with these
names.
Prefixed the internal names with underscores.
An alternative constant _size_constant is provided for use in C++98,
for example for declaring arrays.
Also renamed underlying_traits to integral_mapping.
With this change, the underlying type can be a non-integral type that
provides conversions to and from an integral type. See the test at
test/cxxtest/underlying.h for some examples - though they are more
verbose than strictly necessary, for testing needs.
Move constructors in underlying types are not supported. It has been
difficult so far to get constexpr code not to select the move
constructor, which is generally not constexpr, for various operations.
