Use hardhat-exposed plugin to remove most mocks

[frangio]

We are looking into removing mock contracts to make maintainability easier. The hardhat-exposed plugin I built automatically creates "mock" contracts that 1) expose internal functions as external functions, and 2) add a constructor with the necessary parameters to make the contract concrete (unless it has unimplemented functions).

The "exposed" contracts have an X prefix, and the automatic external functions have an x prefix. I would've liked to use a $ prefix for both but this is currently not supported by Ethers so I went with x, although since our test suite doesn't use Ethers we could go with $ if I make the plugin configurable.

Some mocks need to remain but we can make them simpler by leaving it to the plugin to expose the internal functions. For more detail refer to the readme of hardhat-exposed.

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In this PR I also included the removal of chainId functions in the mock contracts. We don't need this anymore because solidity-coverage runs in Hardhat EVM now.

[frangio]

Compilation times double with this plugin. Currently it increases in this repo from 5 seconds to 11 or so. This will likely go down once we remove most of the manual mocks.

[frangio]

Comments by @Amxx:

In some cases we want to test a modifier (see AccessControlMock) Overloaded functions (like in SafeMath) are difficult test How to test StorageSlot, also see ArraysImpl

[frangio]

Closing in favor of #3666