Updates feature gate to use new_zeroed_alloc

[wasv]

Needed in order to build with newest nightly

new_zeroed_alloc is a more specific version of new_uninit as introduced in:

https://github.com/rust-lang/rust/issues/129396

P.S. I only noticed this because I'm trying to build dust for NixOS. Any interest in a PR to put the NixOS package definitions alongside the source itself? It'd be a nix subdirectory, a flake.nix and a flake.lock that'd be added.