u128 array zero fill - quick solution

[brody2consult]

This is a quick solution to support the case I described in #594:

pub fn f() -> [u128; 2] { return [0; 2] }

This proposal would help me get past the first build issue I have encountered with RustCrypto.

I would love to support non-zero memset fill for both u128 & i128 as discussed in #594, but this would involve some more work in testing to hopefully ensure that this is done correctly & not likely broken by potential future refactoring.

I would be happy to address any possible feedback on my first contribution to Rust-GPU.

TODO

• squash commits before merging - Reviewers please let me know if you want me to do this or if you want to do this yourself