Added new lint for map_or(..., identity)

[teofr]

Introduce a new lint for mapping the identity function with map_or over Option and Result.

Lints:

fn foo(opt: Option<i32>, default: i32) -> i32 { opt.map_or(default, |o| o) }

Into

fn foo(opt: Option<i32>, default: i32) -> i32 { opt.unwrap_or(default) }

Fixes #15801

We had some discussions with @ada4a about naming on issue #15801 , a summary here.

There's a question about naming for this lint, there's already unnecessary_option_map_or_else and unnecessary_result_map_or_else, but I think having two lints doesn't make sense in this case.
unnecessary_map_or could be a reasonable name for this new lint, but it's already taken

changelog: [map_or_identity]: new lint

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