AK+Userland+Kernel+Tests: Deduplicate IPv4 header checksumming

[implicitfield]

Previously, there were two slightly different implementations of this in the Kernel and in LibCrypto.

This patch removes those, and replaces them with an implementation that seeks to combine the best aspects of both while not being a drop-in replacement for either.

Two new tests are also included.

---

FWIW I'm not sure why the old kernel-side implementation had this logic in the core loop:

if (m_checksum & 0x80000000)
    m_checksum = (m_checksum & 0xffff) | (m_checksum >> 16);

That kind of looks like an attempt to prevent unsigned 32-bit integer overflow, but I can't see that realistically happening (you'd have to sum up at least 131075 (UINT32_MAX/UINT16_MAX*2+1) bytes before that'd happen, and even that's a worst-case scenario). The example C implementation in RFC1071 does no such thing either. It seems like this part originated from the old LibC internet_checksum function, so I guess it can go.

---

This also gets rid of all the endianness conversions in the checksumming process (since the checksum is actually byte-order independent, see 2.B of the aforementioned RFC).