1. Expose value_type and difference_type (where present) to user's code.
2. Prohibit arithmetic operations on pointers to non-object types. In
particular, arithmetic operations such as fetch_add/fetch_sub will no longer
compile for pointers to cv void, pointers to functions and pointers to
non-static class members.
Also, use C++11 <type_traits> when possible instead of Boost.TypeTraits to
reduce dependencies. Cleaned up value_arg_type internal type usage for more
efficient argument passing.
