add Allocation::name() getter

[Firestar99]

What the title says, nothing special. I don't want to store yet another copy of the same string, it would be nice if I could just query it instead.

[MarijnS95]

Should the PR title be prefixed with vulkan: or are you intending to add this to the other backends (dx12 and Metal) as well?

[Jasper-Bekkers]

Looking at this, does anyone know why it's name: Option<Box<str>> and not name: Option<String>?

[MarijnS95]

@Jasper-Bekkers that should be #70, which explains it's a minor reduction in struct size.