clippy: Use write! rather than push_str, format

This does involve unwrap, but of course that can't fail unless the
formats fail, which would already panic (that's implied by format!).

crates/arti/src/lib.rs

@@ -122,6 +122,8 @@ pub mod process;
 pub mod socks;
 pub mod watch_cfg;
 
+use std::fmt::Write;
+
 pub use cfg::{
     ApplicationConfig, ApplicationConfigBuilder, ArtiConfig, ArtiConfigBuilder, ProxyConfig,
     ProxyConfigBuilder, SystemConfig, SystemConfigBuilder,
@@ -267,7 +269,7 @@ pub fn main_main() -> Result<()> {
         // If we couldn't resolve the default config file, then too bad.  If something
         // actually tries to use it, it will produce an error, but don't fail here
         // just for that reason.
-        config_file_help.push_str(&format!(" Defaults to {:?}", default));
+        write!(config_file_help, " Defaults to {:?}", default).unwrap();
     }
 
     // We create the runtime now so that we can use its `Debug` impl to describe it for

crates/tor-cell/src/relaycell/msg.rs

@@ -8,6 +8,7 @@ use crate::chancell::msg::{DestroyReason, TAP_C_HANDSHAKE_LEN, TAP_S_HANDSHAKE_L
 use crate::chancell::CELL_DATA_LEN;
 use caret::caret_int;
 use educe::Educe;
+use std::fmt::Write;
 use std::net::{IpAddr, Ipv4Addr};
 use tor_bytes::{Error, Result};
 use tor_bytes::{Readable, Reader, Writeable, Writer};
@@ -921,9 +922,9 @@ impl Resolve {
                 for o in v6.octets().iter().rev() {
                     let high_nybble = o >> 4;
                     let low_nybble = o & 15;
-                    s.push_str(&format!("{:x}.{:x}.", low_nybble, high_nybble));
+                    write!(s, "{:x}.{:x}.", low_nybble, high_nybble).unwrap();
                 }
-                s.push_str("ip6.arpa");
+                write!(s, "ip6.arpa").unwrap();
                 s
             }
         };

crates/tor-dirclient/src/util.rs

@@ -1,16 +1,20 @@
 //! Helper functions for the directory client code
 
+use std::fmt::Write;
+
 /// Encode an HTTP request in a quick and dirty HTTP 1.0 format.
 pub(crate) fn encode_request(req: &http::Request<()>) -> String {
     let mut s = format!("{} {} HTTP/1.0\r\n", req.method(), req.uri());
 
     for (key, val) in req.headers().iter() {
-        s.push_str(&format!(
+        write!(
+            s,
             "{}: {}\r\n",
             key,
             val.to_str()
                 .expect("Added an HTTP header that wasn't UTF-8!")
-        ));
+        )
+        .unwrap();
     }
     s.push_str("\r\n");
     s