Fix current build warnings

This changes the minimum supported rustc, and that can be documented in
CI if necessary

.github/workflows/main.yml

@@ -1,6 +1,5 @@
 name: CI
 on: [push, pull_request]
-
 jobs:
   test:
     name: Test

README.md

@@ -13,7 +13,3 @@ Add this to your `Cargo.toml`:
 [dependencies]
 getopts = "0.2"
 ```
-
-## Rust Version Support
-
-The minimum supported Rust version is 1.18.

src/lib.rs

@@ -503,15 +503,15 @@ impl Options {
 
     /// Derive a custom formatted message from a set of options. The formatted options provided to
     /// a closure as an iterator.
-    pub fn usage_with_format<F: FnMut(&mut Iterator<Item = String>) -> String>(
+    pub fn usage_with_format<F: FnMut(&mut dyn Iterator<Item = String>) -> String>(
         &self,
         mut formatter: F,
     ) -> String {
         formatter(&mut self.usage_items())
     }
 
     /// Derive usage items from a set of options.
-    fn usage_items<'a>(&'a self) -> Box<Iterator<Item = String> + 'a> {
+    fn usage_items<'a>(&'a self) -> Box<dyn Iterator<Item = String> + 'a> {
         let desc_sep = format!("\n{}", repeat(" ").take(24).collect::<String>());
 
         let any_short = self.grps.iter().any(|optref| !optref.short_name.is_empty());