examples: avoid format collect

Fixes:
```
error: use of `format!` to build up a string from an iterator
  --> rcgen/examples/rsa-irc.rs:30:25
   |
30 |     let hash_hex: String = hash.as_ref().iter().map(|b| format!("{:02x}", b)).collect();
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: call `fold` instead
  --> rcgen/examples/rsa-irc.rs:30:46
   |
30 |     let hash_hex: String = hash.as_ref().iter().map(|b| format!("{:02x}", b)).collect();
   |                                                 ^^^
help: ... and use the `write!` macro here
  --> rcgen/examples/rsa-irc.rs:30:54
   |
30 |     let hash_hex: String = hash.as_ref().iter().map(|b| format!("{:02x}", b)).collect();
   |                                                         ^^^^^^^^^^^^^^^^^^^^
   = note: this can be written more efficiently by appending to a `String` directly
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#format_collect
   = note: `-D clippy::format-collect` implied by `-D warnings`
```

Author: cpu

rcgen/examples/rsa-irc-openssl.rs

@@ -2,6 +2,7 @@
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
 	use rcgen::{date_time_ymd, Certificate, CertificateParams, DistinguishedName};
+	use std::fmt::Write;
 	use std::fs;
 
 	let mut params: CertificateParams = Default::default();
@@ -21,7 +22,10 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 	let pem = pem::parse(&pem_serialized)?;
 	let der_serialized = pem.contents();
 	let hash = ring::digest::digest(&ring::digest::SHA512, der_serialized);
-	let hash_hex: String = hash.as_ref().iter().map(|b| format!("{b:02x}")).collect();
+	let hash_hex = hash.as_ref().iter().fold(String::new(), |mut output, b| {
+		let _ = write!(output, "{b:02x}");
+		output
+	});
 	println!("sha-512 fingerprint: {hash_hex}");
 	println!("{pem_serialized}");
 	println!("{}", cert.serialize_private_key_pem());

rcgen/examples/rsa-irc.rs

@@ -6,6 +6,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 	use rsa::RsaPrivateKey;
 
 	use rcgen::{date_time_ymd, Certificate, CertificateParams, DistinguishedName};
+	use std::fmt::Write;
 	use std::fs;
 
 	let mut params: CertificateParams = Default::default();
@@ -27,7 +28,10 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 	let pem = pem::parse(&pem_serialized)?;
 	let der_serialized = pem.contents();
 	let hash = ring::digest::digest(&ring::digest::SHA512, der_serialized);
-	let hash_hex: String = hash.as_ref().iter().map(|b| format!("{:02x}", b)).collect();
+	let hash_hex = hash.as_ref().iter().fold(String::new(), |mut output, b| {
+		let _ = write!(output, "{b:02x}");
+		output
+	});
 	println!("sha-512 fingerprint: {hash_hex}");
 	println!("{pem_serialized}");
 	println!("{}", cert.serialize_private_key_pem());