[`str::split`](https://doc.rust-lang.org/std/primitive.str.html#method.split)allocates a vector and generates considerably more instructions when compiledthan [`str::split_once`](https://doc.rust-lang.org/std/primitive.str.html#method.split_once).[`u64::from_str_radix(split_lo, 16)`](https://doc.rust-lang.org/std/primitive.u64.html#method.from_str_radix)will error if the `lsn_str` contains more than one `/` so this change shouldresult in the same behavior as the current implementation despite not explicitlychecking this.