Refactor implementation of unnecessary_{option,result}_map_or_else

[teofr]

When checking if a function is an identity function, try to use existing functionalities.

Some cases are no longer covered, but I believe that's ok and there were some bugs:

• No longer considering the identity a closure where the pattern captures by reference, since that made the suggestion break compilation
• No longer going into other function's body to check if they're isomorphic to the identity, discussed a bit [on Zulip](#clippy > Going into other function definitions desirable?). I understand this could not catch some cases, but I think it's something that is out the scope of this lint. It also requires some ad hoc mechanism in the implementation, doable but a bit hacky.

Fixes #15916

changelog: [unnecessary_option_map_or_else]: function definitions are no longer traversed when checking if an expression is the identity
changelog: [unnecessary_result_map_or_else]: function definitions are no longer traversed when checking if an expression is the identity

[qmwrygdxpzc]

While you are at it, consider making the doc adhere to the template.

diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index c9066be51..fe8990ef0 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -4642,6 +4642,7 @@ declare_clippy_lint! {
 }
 
 declare_clippy_lint! {
+    /// ### What it does
     /// Checks for usage of `.map_or_else()` "map closure" for `Option` type.
     ///
     /// ### Why is this bad?

[rustbot]

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