Use SipHash-2-4 for CCoinsCache index

This is ~1.7x slower than the Lookup3-of-Xor-with-salt construct we were
using before, but it is a primitive designed for exactly this.

src/uint256.cpp

@@ -80,67 +80,3 @@ template std::string base_blob<256>::GetHex() const;
 template std::string base_blob<256>::ToString() const;
 template void base_blob<256>::SetHex(const char*);
 template void base_blob<256>::SetHex(const std::string&);
-
-static void inline HashMix(uint32_t& a, uint32_t& b, uint32_t& c)
-{
-    // Taken from lookup3, by Bob Jenkins.
-    a -= c;
-    a ^= ((c << 4) | (c >> 28));
-    c += b;
-    b -= a;
-    b ^= ((a << 6) | (a >> 26));
-    a += c;
-    c -= b;
-    c ^= ((b << 8) | (b >> 24));
-    b += a;
-    a -= c;
-    a ^= ((c << 16) | (c >> 16));
-    c += b;
-    b -= a;
-    b ^= ((a << 19) | (a >> 13));
-    a += c;
-    c -= b;
-    c ^= ((b << 4) | (b >> 28));
-    b += a;
-}
-
-static void inline HashFinal(uint32_t& a, uint32_t& b, uint32_t& c)
-{
-    // Taken from lookup3, by Bob Jenkins.
-    c ^= b;
-    c -= ((b << 14) | (b >> 18));
-    a ^= c;
-    a -= ((c << 11) | (c >> 21));
-    b ^= a;
-    b -= ((a << 25) | (a >> 7));
-    c ^= b;
-    c -= ((b << 16) | (b >> 16));
-    a ^= c;
-    a -= ((c << 4) | (c >> 28));
-    b ^= a;
-    b -= ((a << 14) | (a >> 18));
-    c ^= b;
-    c -= ((b << 24) | (b >> 8));
-}
-
-uint64_t uint256::GetHash(const uint256& salt) const
-{
-    uint32_t a, b, c;
-    const uint32_t *pn = (const uint32_t*)data;
-    const uint32_t *salt_pn = (const uint32_t*)salt.data;
-    a = b = c = 0xdeadbeef + WIDTH;
-
-    a += pn[0] ^ salt_pn[0];
-    b += pn[1] ^ salt_pn[1];
-    c += pn[2] ^ salt_pn[2];
-    HashMix(a, b, c);
-    a += pn[3] ^ salt_pn[3];
-    b += pn[4] ^ salt_pn[4];
-    c += pn[5] ^ salt_pn[5];
-    HashMix(a, b, c);
-    a += pn[6] ^ salt_pn[6];
-    b += pn[7] ^ salt_pn[7];
-    HashFinal(a, b, c);
-
-    return ((((uint64_t)b) << 32) | c);
-}