dev: Felt252DictTrait::get could return a @T rather than T for more polivalence

[tdelabro]

Problem

Returning T requires adding a +Copy<T> constraint which forces the user to go call entry and finalize for non-copy types, which is a painful DevX.

Solution

Returning a snapshot of the value, rather than the value itself.
The caller has one more degree of liberty, he can decide to never desnap it if he has no reason to pay this price.

Related code

Rust Slice::get returns a reference to the value, not the value itself.
Same for HashMap::get

Will I create a PR?

Here is a draft PR that does exactly that: #5667
I wrote it to make sure there were no huge blockers down the road. It looks like it works perfectly.
Tell me if you wish to merge it.

[tdelabro]

@orizi I closed the draft PR as you asked, but the issue stays relevant.

Because it is a breaking change, it can't be merged easily. How do you think we should proceed?