Initialize MAX_UINT more neatly

[barakman]

Use 2 ^ 256 - 1 instead that huge constant value.

Fixes #

🚀 Description

Initialize MAX_UINT in a manner which makes it clear why this is indeed the maximum uint value.

• [x ] 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• [x ] ✅ I've added tests where applicable to test my new functionality.
• [x ] 📖 I've made sure that my contracts are well-documented.
• [x ] 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[nventuro]

Wow, I wonder how I missed this, thanks @barakman!

[barakman]

@nventuro:

You're welcome.
I've got a few more of those (mostly submitted by now).
Up until now, I used to fix them locally on my repo, but it's becoming harder and harder for me to merge changes from OpenZeppelin repo.

[nventuro]

Why is it becoming harder for you? Let me know how I can be of assistance to help you further contribute.

[barakman]

@nventuro:
Harder for me to merge your code into my repo, only as long as I apply all these fixes locally.
Once I can get you to apply them on your side (assuming those fixes are correct), then it becomes much easier for me (i.e., more or less a pure copy-paste).