feat: Add Constants library

[CodeSandwich]

Fixes #267. Gathers up all constants used throughout forge-std in a single library so they are easier to access.

The refactoring around the constants is minimal, it's mostly replacements of arbitrary values with references to the new library. No changes are visible from the outside, the 3 constants removed from StdUtil were private. Many more refactorings, like dropping other private constants that are used more than once or are variations about type(uintXXX).max, may be done in separate PRs.

[CodeSandwich]

Well, it doesn't compile with 0.6.2 as it doesn't allow assigning constants to constants, which defeats one of two purposes of this PR. A library with the constants is still useful for the forge-std users, but there's no way to deduplicate the constants inside forge-std without introducing breaking changes.

What should we do now?

1. Only introduce the library, but don't use it internally.
2. Drop support for Solidity 0.6.2 (It's a 5 years old compiler which has a smooth path of upgrade to the 4.5 years old 0.6.12. Does anybody use the 0.6 family for active development with modern tools in 2025?).
3. Close this PR.

Edit: It's much worse, it breaks on anything below 0.8.16 which explicitly introduced assignment of library constants to other constants. I guess that the only way forward is to introduce the library and not use it internally until we drastically increase the minimum compiler version.

[zerosnacks]

Hi @CodeSandwich, thanks for your PR

Why is inheriting the CommonBase contract where you want access to the constants an issue?

Given the described limitations I would prefer not duplicating these constants

Appreciate the clarifying fix on Base.sol, makes sense

[CodeSandwich]

Why is inheriting the CommonBase contract where you want access to the constants an issue?

It's discussed in #267. Inheritance is a barrier when accessing these constants, it excludes usage in libraries and free functions, which are excellent ways to create helpers and utilities for tests and scripts.

Free constants are another option, but they tend to pollute namespaces and they don't work with Solidity older than 0.7.4.

Expressing constants as free functions could work, but they would be clunky they wouldn't work with Solidity older than 0.7.1.