Use stricter paths when referring to items

https://github.com/althonos/blanket/blob/ce0b8ab848769680d2f290bf1d000c2dbbb29d8a/src/types/arc.rs#L11-L13

Just a couple small tips/recommendations:

It'd probably be better to use e.g. `::std::sync::Arc` instead of `std::sync::Arc` here (and likewise for all of the other types), otherwise the path refers to whatever is named `std` in the local scope (which isn't necessarily the `std` crate).

https://doc.rust-lang.org/reference/paths.html#path-qualifiers

Further these types should be pulled from `::core` else `::alloc` (if not found in core) as these crates are simply re-exported by `std` but have the additional benefit of being supported/accessible in `no_std` crates.