Fully qualify memset and memcpy

And other trivial touches.

Ispired by Lucas's DiscoCheck

No functional change.

src/rkiss.h

@@ -43,14 +43,12 @@
 
 class RKISS {
 
-  // Keep variables always together
-  struct S { uint64_t a, b, c, d; } s;
+  struct S { uint64_t a, b, c, d; } s; // Keep variables always together
 
   uint64_t rotate(uint64_t x, uint64_t k) const {
     return (x << k) | (x >> (64 - k));
   }
 
-  // Return 64 bit unsigned integer in between [0, 2^64 - 1]
   uint64_t rand64() {
 
     const uint64_t