Expose unsafe `into_parts` and `from_parts` functions

[Bahex]

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.

[laurmaedje]

I would be fine with having this, if you want to make a PR.