keccak: ARMv8 asm! cleanups (#101)

We pass in `state.as_mut_ptr()` as `x0`, which was previously marked as
`in`. However, it's modified in the `asm!`, e.g.:

    sub x0, x0, #192

Note: it's also post-incremented by `ld1.1d` as state is loaded.

Since it's modified, it needs to be `inout`. This marks it as such,
using the `=> _` syntax to discard the result, then it's reset, then
modified as state is written).

Similar mutations were occurring with `x1` and `x8` as well, which have
been modified accordingly.

keccak/src/armv8.rs

@@ -124,9 +124,9 @@ pub unsafe fn p1600_armv8_sha3_asm(state: &mut [u64; 25], round_count: usize) {
             st1.1d	{{v20-v23}}, [x0], #32
             st1.1d	{{v24}},     [x0]
         ",
-            in("x0") state.as_mut_ptr(),
-            in("x1") crate::RC[24-round_count..].as_ptr(),
-            in("x8") round_count,
+            inout("x0") state.as_mut_ptr() => _,
+            inout("x1") crate::RC[24-round_count..].as_ptr() => _,
+            inout("x8") round_count => _,
             clobber_abi("C"),
             options(nostack)
         );