perf(span): hash `Span` as a single `u64` (#8299)
#8298 made `Span` aligned on 8 on 64-bit platforms. Utilize this property to hash `Span` as a single `u64` instead of 2 x `u32`s. This reduces hashing a `Span` with `FxHash` to 3 instructions (down from 7), and 1 register (down from 3). https://godbolt.org/z/4q36xrWG8