firmware: simplify some `use`s.

artiq/firmware/libproto_artiq/mgmt_proto.rs

@@ -1,4 +1,4 @@
-use alloc::vec::Vec;
+use alloc::Vec;
 #[cfg(feature = "log")]
 use log;
 

artiq/firmware/libproto_artiq/session_proto.rs

@@ -1,5 +1,4 @@
-use alloc::vec::Vec;
-use alloc::string::String;
+use alloc::{Vec, String};
 
 use io::{Read, Write, Error, Result};
 use io::proto::{ProtoRead, ProtoWrite};