Simplify Initializable

[frangio]

In #3344 we introduced changes to Initializable in order to enable the use of _disableInitializers() in the constructor of multiple parents:

contract A {
    constructor() { _disableInitializers(); }
}
contract B {
    constructor() { _disableInitializers(); }
}
contract C is A, B {
    constructor() { _disableInitializers(); }
}

However, the resulting code was difficult to reason about, and it introduced a bug when mixing _disableInitializers and the old pattern constructor() initializer {}.

We sat down to rethink it and trim it down to more straightforward code.

This PR slightly changes how functions with the initializer and reinitializer modifiers can be nested in the context of the constructor.

• initializer functions work the same as 4.6: they can be nested in this context. This is present for backwards compatibility with versions prior to 4.4.1 and we may remove it in 5.0.
• reinitializer functions can't be nested. If one is invoked in the context of another, it will revert. This is a small breaking change for reinitializer(1). We don't think anyone will be affected by this change given that reinitializers were released only 1 month ago and the broken pattern was not a recommended one. It allows us to make the code much simpler.