Ensure ConstPointers don't outlive parent

[justsmth]

Description of changes:

• Use lifetimes to prevent a "child" object being used after its "parent" is out of scope.
• Move most non-mutating functionality from LcPtr<EVP_PKEY> to ConstPointer<'_, EVP_PKEY>

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 78.54077% with 50 lines in your changes missing coverage. Please review.

Project coverage is 92.39%. Comparing base (c358484) to head (90f0ca9).
Report is 218 commits behind head on main.

Files with missing lines	Patch %	Lines
aws-lc-rs/src/ed25519.rs	64.70%	0 Missing and 12 partials ⚠️
aws-lc-rs/src/ec.rs	57.89%	3 Missing and 5 partials ⚠️
aws-lc-rs/src/ec/encoding.rs	65.00%	3 Missing and 4 partials ⚠️
aws-lc-rs/src/agreement.rs	66.66%	1 Missing and 3 partials ⚠️
aws-lc-rs/src/evp_pkey.rs	93.61%	2 Missing and 1 partial ⚠️
aws-lc-rs/src/rsa/key.rs	76.92%	0 Missing and 3 partials ⚠️
aws-lc-rs/src/pqdsa/key_pair.rs	60.00%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/pqdsa/signature.rs	0.00%	0 Missing and 2 partials ⚠️
aws-lc-rs/src/ptr.rs	94.59%	2 Missing ⚠️
aws-lc-rs/src/rsa/encoding.rs	60.00%	0 Missing and 2 partials ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #693      +/-   ##
==========================================
- Coverage   95.80%   92.39%   -3.41%     
==========================================
  Files          61       71      +10     
  Lines        8143     9061     +918     
  Branches        0     9061    +9061     
==========================================
+ Hits         7801     8372     +571     
- Misses        342      423      +81     
- Partials        0      266     +266     

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