Proofs for Vec::swap_remove, Option::as_slice, and VecDeque::swap

[stogaru]

Resolves: #76

Changes

• Adds proofs for the following functions using raw pointer operations:
  • Vec::swap_remove
  • Option::as_slice
  • VecDeque::swap
• ideally the usages should have been verified by stubbing the contracts for reaw pointer operations like byte_add, add and offset, but stubbing cannot be done for these functions at this time due to Compilation Error When Stubbing Functions That Return Pointers Using Function Contracts kani#3732
• Marks Challenge 3 as Resolved and changes its end date.
• Adds contributors.

PoCs:

• Vec::swap_remove: @MayureshJoshi25
• Option::as_slice, VecDeque::swap: @stogaru

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[feliperodri]

@stogaru Update the description of the PR to close the issue, update the goal's document stating the challenge is completed, and list the previous PRs that need to be merged before we can evaluate this PR and complete the goal.

[stogaru]

@feliperodri, I made the required changes.