Safer pointer dereferences

[justsmth]

Description of changes:

• Makes acquiring a mutable or const pointer from a ManagedPointer explicit (instead of using Deref):
  • Adds a MutPointer type as an analog of ConstPointer.
  • On a mutable ManagedPointer use as_mut to obtain a MutPointer.
  • On a shared ManagedPointer use as_const to obtain a ConstPointer.
  • On a shared ManagedPointer use as_mut_unsafe to obtain a MutPointer.

Call-outs:

The uses of as_mut_unsafe are limited to calls made to the following AWS-LC functions:

• EVP_PKEY_CTX_new
• EVP_PKEY_up_ref
• EVP_DigestSignInit
• EVP_DigestVerifyInit

These function only modify EVP_PKEY ref-count while holding a global lock.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.63636% with 47 lines in your changes missing coverage. Please review.

Project coverage is 92.52%. Comparing base (c358484) to head (8c8e522). Report is 78 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/ec.rs	63.82%	0 Missing and 17 partials :warning:
aws-lc-rs/src/agreement.rs	80.00%	0 Missing and 5 partials :warning:
aws-lc-rs/src/rsa/key.rs	70.58%	0 Missing and 5 partials :warning:
aws-lc-rs/src/rsa/encryption/pkcs1.rs	66.66%	0 Missing and 4 partials :warning:
aws-lc-rs/src/kem.rs	72.72%	0 Missing and 3 partials :warning:
aws-lc-rs/src/rsa/signature.rs	57.14%	0 Missing and 3 partials :warning:
aws-lc-rs/src/ec/key_pair.rs	71.42%	0 Missing and 2 partials :warning:
aws-lc-rs/src/evp_pkey.rs	90.00%	2 Missing :warning:
aws-lc-rs/src/rsa/encoding.rs	60.00%	0 Missing and 2 partials :warning:
aws-lc-rs/src/rsa/encryption/oaep.rs	89.47%	0 Missing and 2 partials :warning:
... and 2 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #505 +/- ## ========================================== - Coverage 95.80% 92.52% -3.28% ========================================== Files 61 67 +6 Lines 8143 9277 +1134 Branches 0 9277 +9277 ========================================== + Hits 7801 8584 +783 - Misses 342 422 +80 - Partials 0 271 +271 ```

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