CachedBitGenerator: value_type -> result_type to fix for libcxx
https://reviews.llvm.org/D120630 introduced a __libcpp_random_is_valid_urng check which requires return value to be in ::result_type (sidenote: that check goes beyond enforcing https://en.cppreference.com/w/cpp/numeric/random/uniform_random_bit_generator). Looking at an arbitrary generator in the standard library https://en.cppreference.com/w/cpp/numeric/random/independent_bits_engine , it also provides result_type, not value_type.