Do not explicitly set rkyv pointer size

[arbor-arthur]

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?