extend array len to 64bits

[kbkpbot]

I saw in ROADMAP.md said that [x] 64/32 bit int depending on arch (will remove array.len limitation on 64 bit systems).
But I can't find the implementation in current V.

So, what will it look like? usize? isize? i64? u64?

[spytheman]

int will behave like isize