Fix lint manual_slice_size_calculation when a slice is ref more than once

[Coekjan]

When a slice is ref more than once, current suggestion given by manual_slice_size_calculation is wrong. For example:

let s: &[i32] = &[1, 2][..];
let ss: &&[i32] = &s;  // <-----

let _ = size_of::<i32>() * ss.len();

clippy now suggests:

- let _ = size_of::<i32>() * ss.len();
+ let _ = size_of_val(ss);

However, this can result in calculating the size of &[i32], instead of [i32] (this wrong suggestion also leads to size_of_ref warning: https://rust-lang.github.io/rust-clippy/master/index.html#/size_of_ref )

Now I am sending this PR to fix this bug, so that clippy will suggest (some deref added):

- let _ = size_of::<i32>() * ss.len();
+ let _ = size_of_val(*ss);

As I am not familiar with current clippy code-base, please correct me if I am not doing well or I can do it better :)

changelog: [manual_slice_size_calculation]: fix a bug when a slice is ref more than once.

[rustbot]

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[dswij]

LGTM, thanks!

[dswij]

@bors r+

[bors]

📌 Commit 2080634 has been approved by dswij

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2080634 with merge 6a281e9...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing 6a281e9 to master...