`static-alloc`: Implement a speedy version of `alloc_zeroed`

In a special case, when the original constructor was `zeroed`, then the function does not need to perform its own zeroing of memory. This is helped by the fact that there is no reset function.

This could be implemented by a simple `bool` attribute containing the information on whether all of its remaining memory is zeroed.

Look at the `static-alloc` crate's source, particularly the `alloc_zeroed` function and the data structures that track memory state. The issue suggests adding a boolean attribute to track if remaining memory is already zeroed. Start by understanding how memory allocation and zeroing currently work, then modify the relevant struct and the `alloc_zeroed` implementation to skip zeroing when appropriate. Run existing tests to ensure correctness.

Written by the indexing model from the issue text.