Provide explicit impl Hash for KeyData

[zopsicle]

A derived Hash impl would call write_u32 twice. We call write_u64 once, which is beneficial if the hasher implements write_u64 explicitly.

Example disassembly when using rustc_hash:

use rustc_hash::FxHasher;

pub fn f(k: &KeyData) -> u64
{
    let mut h = FxHasher::default();
    k.hash(&mut h);
    h.finish()
}

# before
mov     eax, dword ptr [rdi]
movabs  rcx, 5871781006564002453
imul    rax, rcx
rol     rax, 5
mov     edx, dword ptr [rdi + 4]
xor     rax, rdx
imul    rax, rcx
ret

# after
movabs  rax, 5871781006564002453
imul    rax, qword ptr [rdi]
ret