`HybridUint` for mix-match `Uint` in-struct

This could be a variant of "worst case" where the Limbs are MaybeUninit or heapless::Vec which uses MaybeUninit behind the scenes. This would allow easier portability between BoxedUint and no-alloc variant without re-writing into fixed sizes whilst avoiding global alloc ? e.g. here: - https://github.com/RustCrypto/RSA/pull/636#discussion_r2715157271 Say there is: ```rust struct Foo { a: HeaplessUint<2>, // U64/U128 Max b: HeaplessUint<1>, // U32/64 Max } ``` It would be more memory efficient than: ```rust struct Foo<U: Unsigned> { a: U b: U } ``` Where used `U` can be `U2048` | `U4096` etc. in case which it takes double the space vs optimising through worst case. And more flexible than relying on type-aliases: ```rust struct Foo { a: U2048, b: U128, } ``` Arguably it is just another form of `Uint<Limbs>` but Heapless is essentially optimizing through `[MaybeUninit<Limb>; N]`