Do not explicitly set rkyv pointer size

Cargo.toml currently sets `rkyv-serialize-no-std = ["rkyv/size_32"]` which prevents using `rkyv` with any other pointer size. It is best practice for libraries to not set the pointer size and allow binaries to choose the pointer size themselves. For myself, I patched this to `rkyv-serialize-no-std = ["rkyv"]`, although I imagine that testing might require it to still be set in some circumstances?