Static assert arithmetic types in basic radix sort API

As part of https://github.com/NVIDIA/cub/pull/671 we introduced support of user-defined types in radix sort. To improve user experience, we should static assert that the key type is supported by `cub::Traits` (`detail::radix::is_fundamental_type::value`) and direct users to `DecomposerT` overloads otherwise.

In order to make the error message readable, we should also follow the pattern of `detail::radix::decomposer_check_t` and SFINAE out trampoline function to avoid instantiating dispatch layer.

Start by reading PR #671 and the basic radix sort API, then trace cub::Traits, detail::radix::is_fundamental_type, and decomposer_check_t through the trampoline and dispatch layer. Done means unsupported key types produce a readable static assertion directing users to DecomposerT overloads without instantiating the dispatch layer.

Written by the indexing model from the issue text.