Use std::fmt::radix instead of to_str_radix.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
std: Add some implementation of common traits

- semver::Version is now Eq, Ord, and Hash
- Path is now PartialOrd and Ord
auto merge of #15564 : alexcrichton/rust/moar-hash, r=huonw

- semver::Version is now Eq, Ord, and Hash
- Path is now PartialOrd and Ord
auto merge of #15570 : omasanori/rust/radix, r=alexcrichton