Refactor imports for contracts from `solidity-bignumber` external library

[andrejrakic]

Why

With the current setup, it can be confusing to developers where the imported contracts are coming from. One can misinterpret that solidity-bignumber contracts are coming from the external NPM package instead of from the local vendor folder if one doesn't check the remappings.txt file, which is a real possibility. Check this comment for reference.

What

• [x] Refactor all contracts where import "@zondax/solidity-bignumber/*"; appears to relative imports to the vendor/solidity-BigNumber folder.
• [x] Delete the @zondax/solidity-bignumber/=vendor/solidity-BigNumber/ line from the remappings.txt file.
• [x] Make sure that the whole project compiles (you can run smoke_tests.sh to check)

How

For example, line 26 in MarketMockAPI.sol should become:

import "../../../vendor/solidity-BigNumber/src/BigNumbers.sol";

Who

@vonum

When

Milestone 1

[BlocksOnAChain]

is this ready for review from our side or still in active development? I didn't see any active PRs that are related, so I can't check on my own... @andrejrakic @vonum