Expose unsafe `into_parts` and `from_parts` functions

Expose a way to decompose EcoVec into its components, similar to Vec::into_parts and Vec::from_parts (which are, as of now, nightly only).

This is useful for struct-of-array patterns where multiple vectors always have identical lengths, but might be cloned or mutated separately.

struct SoA {
    foo: EcoVec<Foo>,
    bar: EcoVec<Foo>,
    baz: EcoVec<Baz>,
    qux: EcoVec<Qux>,
}

// [usize; 2 * N] size for N items
assert!(std::mem::size_of::<SoA>() == [usize; 8]);

Providing a way to decompose and recompose EcoVec would make such structs significantly smaller:

struct SoA {
    foo: EcoVecRaw<Foo>,
    bar: EcoVecRaw<Foo>,
    baz: EcoVecRaw<Baz>,
    qux: EcoVecRaw<Qux>,
    len: usize,
}

// [usize; N + 1] size for N items
assert!(std::mem::size_of::<SoA>() == [usize; 5]);

Methods of SoA would recompose the relevant EcoVecRaw (or whatever it ends up being named) with len as necessary to get back an EcoVec.

Start by reading the EcoVec implementation and the linked Vec::into_parts and Vec::from_parts documentation. Determine how EcoVec's components can be safely decomposed and recomposed for the struct-of-array example. Done means the requested unsafe APIs are exposed with behavior that supports storing raw components and restoring EcoVec values.