Add access methods

[mangelats]

Add the methods get, get_mut, get_unchecked, and get_unchecked_mut of Vec to the generated SoA.

Right now it's not possible implement the trait Index and IndexMut. This issue would add a way to access the elements in a way that also works for Vec. This would also make accessing the underlying Vec not required.

// Before
let value = vec.field[1];

// After
let value = vec.get(1)?.field; // vec could be an ExampleVec or a Vec<Example>

And we might get a reference (which we couldn't do before):

let reference = vec.get(2)?;

Also would support ranges:

let slice = vec.get(0..5)?;

And we should probably add ranges to our custom slice so it's possible to get an smaller slice from a slice.

To do so, it would make sense to make a trait similar to SliceIndex but changing the output to not require returning a reference (so we can use our struct of references).

[mangelats]

Really useful for the issue #19 where you don't have underlying vectors, so this gives a common interface for both versions and Vec.

[Luthaf]

Sounds good to me!

[mangelats]

Also adding index and index_mut because get actually returns an Option<T>. It will keep the same behaviour as using the index operator:

// vec: Vec<Example>
vec[0];

// soa: ExampleVec
soa.index(0);

Should behave the same (even when it fails) and work with ranges :)