Update Compatibility Signature Verification Functions

[nlordell]

I was trying to move the signature verification logic to use calldata instead of memory for the signature bytes and decided to make some small changes to the documentation around the legacy checkSignatures functions.

Additionally, since we have additional code size, we were able to move the legacy checkSignatures back into the Safe contract so continued compatibility with existing Safes does not require specific fallback handlers.

[coveralls]

Pull Request Test Coverage Report for Build 8736116277

Details

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.413%

---

Totals
Change from base Build 8617384014:	0.02%
Covered Lines:	398
Relevant Lines:	406

---

💛 - Coveralls