reduce size of Allocation structs

[udoprog]

This changes the name and backtraces fields on Allocation to be Box<str> instead of String, netting us a mem::size_of::<Allocation>() decrease from 104 to 88 bytes.

Same could be done on AllocationError, but would be an API break.

[MarijnS95]

Nice to save 8 bytes per-String on the capcity field, these are "immutable" anyway :)

[manon-traverse]

This looks like it makes sense for gpu-allocator. Thank you very much for your contribution! ^.^