Replace size_t by uint8_t in accessors for the IPv4 header fields
'version' and 'header_length' - uint8_t is the smallest integral type
for 4 bit of information. Note, as the _internet header length_ field is
defined to reflect the number of 32-bit words the header occupies, we
also stick to the specification with our accessor.
Issue #1915
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082