This was not causing any compilation issues, but the solidity language
server gets confused by this and complains about incorrect import
statements otherwise.

If a `MiniMeToken` has a `TokenController` configured, it can intercept
every transfer using the `onTransfer` callback and reenter the
`MiniMeToken` contract.

This is a reentrancy vulnerablity as a malicious `TokenController` has
access to the `MiniMeToken` `balances` and it privileged to perform
transfers.

Unfortunately, simply using the CEI-pattern as done in
#24 isn't sufficient, because the
reentrancy can be done non-recursively, resulting in no error and a
possible double spent issue.

To prevent this vulnerablity, this commit introduces OZ's
`ReentrancyGuard` and makes `MiniMeToken` inherit it. This gives us
access to the `nonReentrant` modifer that is attached to every transfer
function.

The commit also introduces a test that proves that the contract reverts
in case of a reentrancy attempt.

Closes: #17