MTU + bugfixes

• implement MTU setting and querying in the Ethernet module (compatibility with mirage-protocols version 1.1.0), and use this value to inform TCP's MSS. (#288, by @djs55)
• rename the ~payload argument of TCP/UDP marshallers to ~payload_len, in an attempt to clarify that the payload will not be copied to the Cstruct.t returned by these functions (#301, by @talex5)
• functorize ipv6 over a random implementation (#298, by @olleolleolle and @hannesm)
• add tests for sending and receiving UDP packets over IPv6 (#300, by @mattgray)
• avoid float in TCP RTO calculations. (#295, by @olleolleolle and @mattgray)
• numerous bugfixes in header marshallers and unmarshallers (#301, by @talex5 and @yomimono)
• replace polymorphic equality in _packet.equals functions (#302, by @yomimono)