new lints around #[must_use] fns

`must_use_unit` lints unit-returning functions with a `#[must_use]`
attribute, suggesting to remove it.

`double_must_use` lints functions with a plain `#[must_use]`
attribute, but which return a type which is already `#[must_use]`,
so the attribute has no benefit.

`must_use_candidate` is a pedantic lint that lints functions and
methods that return some non-unit type that is not already
`#[must_use]` and suggests to add the annotation.

clippy_lints/src/utils/conf.rs

@@ -90,6 +90,7 @@ macro_rules! define_Conf {
                     }
                 }
 
+                #[must_use]
                 fn $rust_name() -> define_Conf!(TY $($ty)+) {
                     define_Conf!(DEFAULT $($ty)+, $default)
                 }
@@ -153,6 +154,7 @@ define_Conf! {
 }
 
 impl Default for Conf {
+    #[must_use]
     fn default() -> Self {
         toml::from_str("").expect("we never error on empty config files")
     }