Similar to #515 and #519 but with check_pair_func. Reminder of the steps:

1. Review existing tests (see below) and possibly complete them.
2. Refactor
3. Optional: if the refactoring results in more cases being naturally supported, add corresponding test cases.

Step 1:

• Positive: matching key pairs, both normal
• Positive: matching key pairs, private is wrapping
• Negative: non-matching pairs of the same type (different key materials, same bit size or curve)
• Negative: non-matching pairs of the same type (RSA and ECC: different bit/curve size, ECC: same bit size but different curve)
• Negative: non-matching pairs of different types (RSA vs ECC)
• Negative: non-matching pairs (different key material), private is wrapping

Step 2: the usual.

Step 3: currently wrapping (formerly "opaque") RSA keys are not supported, consider adding support for them as it would probably use the same code path as ECC (just with different sizes).