Allow custom storage slots

[ItsNickBarry]

A function like this must be added to each storage library:

function layout(bytes32 slot) internal pure returns (Layout storage l) {
    assembly {
        l.slot := slot
    }
}

[ItsNickBarry]

If we do it like this, we can maintain full test coverage without writing any new tests:

function layout() internal pure returns (Layout storage l) {
    l = layout(STORAGE_SLOT);
}

function layout(bytes32 slot) internal pure returns (Layout storage l) {
    assembly {
        l.slot := slot
    }
}

[ItsNickBarry]

After some discussion, this seems less useful than I originally thought. For it to work, each contract would have to store a reference to the STORAGE_SLOT defined in each library, and access storage via the layout(bytes32) function. The storage slot reference would have to be overridable.

This could still be a useful change, but needs more thought.

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Use cases for custom slots:

• End users maintaining legacy code can override the SS storage slot if SS ever changes its storage slot seed format (solidstate.contracts.storage.xxx).
• Some storage layouts (and contracts) can be reused for different purposes in the same deployment - for example, it may be useful to treat some internal value as an ERC20 and use the ERC20 storage layout for this purpose. This would be most applicable to diamond proxies.

[ItsNickBarry]

Would only work if storage libraries were merged into internal contracts, which is not desirable.

[ItsNickBarry]

I'm once again considering implementing this. If Solidstate introduces a breaking change to support EIP-7201 (#166), we will want to provide a way for existing users to continue using the legacy storage slots.