`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:

struct Foo {
  a: HeaplessUint<2>, // U64/U128 Max
  b: HeaplessUint<1>, // U32/64 Max
}

It would be more memory efficient than:

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:

struct Foo {
  a: U2048,
  b: U128,
}

Arguably it is just another form of Uint<Limbs> but Heapless is essentially optimizing through [MaybeUninit<Limb>; N]

Start by reviewing the existing Uint and HeaplessUint representations, including their use of MaybeUninit, then read the linked RustCrypto/RSA discussion for the portability context. A complete design should support mixed limb capacities within one struct without global allocation while improving on fixed worst-case storage.