Have clippy warn about uninlined format arguments

This makes clippy warn about `format!("{}", var)`, with a
machine-applicable fix converting to `format!("{var}")`.

The `allow-mixed-uninlined-format-args = false` option in `.clippy.toml`
makes clippy issue this warning and provide this fix for individual
format specifiers even if the format string has other specifiers that
can't use inlining.

.clippy.toml

@@ -2,6 +2,7 @@ allow-print-in-tests = true
 allow-expect-in-tests = true
 allow-unwrap-in-tests = true
 allow-dbg-in-tests = true
+allow-mixed-uninlined-format-args = false
 disallowed-methods = [
     { path = "std::option::Option::map_or", reason = "prefer `map(..).unwrap_or(..)` for legibility" },
     { path = "std::option::Option::map_or_else", reason = "prefer `map(..).unwrap_or_else(..)` for legibility" },

Cargo.toml

@@ -80,6 +80,7 @@ string_lit_as_bytes = "warn"
 string_to_string = "warn"
 todo = "warn"
 trait_duplication_in_bounds = "warn"
+uninlined_format_args = "warn"
 verbose_file_reads = "warn"
 wildcard_imports = "warn"
 zero_sized_map_values = "warn"