Add method to read pages of bids

[mi-yu]

• adds getUserDataPage(uint256 limit, uint256 offset) which can be used to page through the bids
• use EnumerableSet to track unique bidder addresses
• note that the order is not guaranteed to be stable for EnumerableSet, so page(offset=0,limit=0) may return different data as more bids are added over time

Coverage

---------------------------|----------|----------|----------|----------|----------------|
File                       |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
---------------------------|----------|----------|----------|----------|----------------|
 contracts/                |    99.04 |    88.24 |    97.59 |    98.61 |                |
  BucketAuction.sol        |    98.63 |    89.02 |    96.77 |       98 |          69,73 |
  DutchAuction.sol         |      100 |    88.64 |      100 |      100 |                |
  ERC721M.sol              |     98.8 |    89.22 |    97.37 |    98.35 |          58,59 |
  ERC721MCallback.sol      |      100 |       70 |      100 |      100 |                |
  IBucketAuction.sol       |      100 |      100 |      100 |      100 |                |
  IDutchAuction.sol        |      100 |      100 |      100 |      100 |                |
  IERC721M.sol             |      100 |      100 |      100 |      100 |                |
  IERC721MCallback.sol     |      100 |      100 |      100 |      100 |                |
 contracts/mocks/          |    76.92 |       50 |     87.5 |       80 |                |
  TestReentrantExploit.sol |       75 |      100 |      100 |       80 |             35 |
  TestStaking.sol          |    77.78 |       50 |       80 |       80 |          25,26 |
---------------------------|----------|----------|----------|----------|----------------|
All files                  |    97.75 |     87.6 |     96.7 |    97.69 |                |
---------------------------|----------|----------|----------|----------|----------------|