Allow custom storage for UnwindContext and Evaluation

[nbdd0121]

Partially addresses #581 (alloc is still a dep, but everything unwind related doesn't require heap).

New traits UnwindContextStorage and EvaluationStorage is introduced to allow customisation of what the storage is. UnwindContext, UnwindTable and Evaluation will gain 1 additional generic parameter which is defaulted to StoreOnHeap.

The ctor with this generic param is called new_in, and the old new method will use the default param so type inference won't fail.

[philipc]

Once we do use const generics, would ArrayVec<[T; N]> become ArrayVec<T, N>? Given that, it doesn't make sense to have ArrayVec<Vec<T>>. I realize it will be more boilerplate, but what do you think of leaving Array and ArrayVec how they are, and adding a new ArrayLike that is implemented for ArrayVec and Vec, so that Vec can be used directly instead of via ArrayVec?

[nbdd0121]

Wouldn't it require exposing ArrayVec to the user?

[nbdd0121]

I think we can still do ArrayVec<[T; N]> with const generics; we don't need the macro anymore and can just do impl<T, const N: usize> ArrayLike for [T; N]> {...}

[philipc]

Ok, that makes sense. So the comment on ArrayLike is wrong: the trait isn't needed due to lack of const generics, only the macro. (The comment was correct prior to this PR.)

[philipc]

Thanks!