[Release][Bugfix] Fix bigunsignedint numeric_limits for clang/libc++

The std::numeric_limits specialization for bigunsignedint requires
access to the internal state of bigunsignedint. Previously, the correct
specialization of std::numeric_limits was a friend of bigunsignedint,
but that creates problems on recent versions of clang with the
alternative libc++ library, because that library declares the base
template of std::numeric_limits as a class and clang subsequently
complains if the friend declaration uses 'struct'. Unfortunately,
libstdc++ uses a struct, making it impossible to keep clang happy for
both standard libraries. So we introduce a helper class that provides
access to the internal state and which now becomes a friend of
bigunsignedint. The numeric_limits specialization inherits from the
helper to use it.