Numeric: Fix Wimplicit-int-float-conversion with clang-10
Numeric: Fix Wimplicit-int-float-conversion with clang-10

Avoid warning by using explicit casting to double

../include/rapidcheck/gen/Numeric.hpp:46:43: warning: implicit conversion from 'std::__1::numeric_limits<unsigned long long>::type' (aka 'unsigned long long') to 'double' changes value from 18446744073709551615 to 18446744073709551616 [-Wimplicit-int-float-conversion]
      (stream.next<uint64_t>() * scale) / std::numeric_limits<uint64_t>::max();
                                        ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

emil-e/rapidcheck,emil-e/rapidcheck,emil-e/rapidcheck