readers: Initial Readers enumerator for detecting YubiKeys

Adds a `yubikey_piv::Readers` type which opens a PC/SC context and can
enumerate detected PC/SC readers with a slightly more ergonomic API than
what's provided in the upstream crate.

Does not support actually instantiating a `YubiKey` from a `Reader<'_>`
yet, but ideally all connections to YubiKeys should go through this API.

src/lib.rs

@@ -156,6 +156,7 @@ mod metadata;
 pub mod mgm;
 #[cfg(feature = "untested")]
 pub mod msroots;
+pub mod readers;
 #[cfg(feature = "untested")]
 mod serialization;
 #[cfg(feature = "untested")]
@@ -164,7 +165,7 @@ mod transaction;
 pub mod yubikey;
 
 #[cfg(feature = "untested")]
-pub use self::{key::Key, mgm::MgmKey};
+pub use self::{key::Key, mgm::MgmKey, readers::Readers};
 pub use yubikey::YubiKey;
 
 /// Object identifiers