miny: Extra padding size

[1e1001]

As suggested in #1:

a const parameter that adds extra bytes of inline space beyond one pointer's worth.

Since I kinda wanna wait until I merge #2 before working on code (to fear of rebase/merge hell), I'll just write down some notes of this and implement it later:

Making it a (hopefully) non-breaking change

My idea is to change the definiton of Miny from:

struct Miny<T: ?Sized> {
  meta: Metadata<T>,
  data: MaybeUninit<*mut ()>,
  marker: PhantomData<T>,
}

to:

struct Miny<T: ?Sized, const S: usize = 1> {
  meta: Metadata<T>,
  data: Data<S>,
  marker: PhantomData<T>,
}
union Data<const S: usize> {
  pointer: *mut (),
  padding: MaybeUninit<[*mut (); S]>,
}

Where S (final name t.b.d.) represents the size of the inline section (in pointer-sizes, to keep things more consistent across platforms), usable without any extra nightly features.

Splitting out the Data would also probably make it easier to delineate safe/unsafe code, as most of the unsafe is just in accessing the combination pointer+value.

Main use for this is as just a small optimization if you know what kind of values you have, e.g. using a Miny<dyn Any, 3> for storing String and Vec -likes inline.

An alternative method could be using a type-generic for referencing the size of a specific type, but I feel like that would break more easily.