Reduce Deploy Cost of All Contracts

[steviezhang]

Changes

• stripped down contracts and created own utils out of openzeppelin utils if it reduced gas cost.
• Removed Ownable from bridge, as we never used the functionality
• Note that the constructor for the bridge costs us 100000 gas
• Updated tests.
• Note that Generic Handler using Handler Helpers resulted in increased gas cost, so it was removed.
• Reduce reason strings

deploy costs changes on this branch

┌─────────┬───────────────────┬─────────┐
│ (index) │       type        │ gasUsed │
├─────────┼───────────────────┼─────────┤
│    0    │     'Bridge'      │ 4081769 │
│    1    │  'ERC20Handler'   │ 1680230 │
│    2    │  'ERC721Handler'  │ 1915811 │
│    3    │ 'GenericHandler'  │ 1411471 │
│    4    │ 'CentrifugeAsset' │ 197353  │
│    5    │ 'HandlerHelpers'  │ 453535  │
│    6    │    'ERC20Safe'    │ 256244  │
│    7    │   'ERC721Safe'    │ 147103  │
└─────────┴───────────────────┴─────────┘

Master

┌─────────┬───────────────────┬─────────┐
│ (index) │       type        │ gasUsed │
├─────────┼───────────────────┼─────────┤
│    0    │     'Bridge'      │ 4164739 │
│    1    │  'ERC20Handler'   │ 1680586 │
│    2    │  'ERC721Handler'  │ 1916167 │
│    3    │ 'GenericHandler'  │ 1467997 │
│    4    │ 'CentrifugeAsset' │ 197353  │
│    5    │ 'HandlerHelpers'  │ 453535  │
│    6    │    'ERC20Safe'    │ 256244  │
│    7    │   'ERC721Safe'    │ 147103  │
└─────────┴───────────────────┴─────────┘

Closes: #162 Closes: #116 Closes: #165