Reducing Safe Contract Code Size

[remedcu]

TODO:

• [x] Using uint256 for v. (Saved around 9 bytes)
• [x] Using storage slot directly where possible. (Saved around 30+ bytes)
• [x] Using if with revert instead of require where possible. (Saved around 20 bytes)
  • [x] Using assembly to revert in Safe.sol and inherited files. (Saved around ~1900 bytes~ 2700 bytes)

Closes #713

[coveralls]

Pull Request Test Coverage Report for Build 7503450091

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 94.397%

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Totals
Change from base Build 7503409586:	-0.2%
Covered Lines:	396
Relevant Lines:	404

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💛 - Coveralls