Rename key module to piv (#277)

Now that the crate is named `yubikey` rather than `yubikey-piv`, it
makes more sense to call this module out as PIV-related functionality.

cli/src/commands/status.rs

@@ -4,7 +4,7 @@ use crate::terminal::STDOUT;
 use gumdrop::Options;
 use std::io::{self, Write};
 use termcolor::{ColorSpec, StandardStreamLock, WriteColor};
-use yubikey::{key::*, YubiKey};
+use yubikey::{piv::*, YubiKey};
 
 use crate::print_cert_info;