ArnaudD-FR

[vector] Allow to iterate with mutables

1

When using mutable buffers it is currently impossible to deference iterators without having a compiler error due to `struct IndirectHelper::Read` returning `return_type` instead of `mutable_return_type`: ```cpp struct VectorIterator { ......

Same as previous commits, use structures when ever possible

# Replace UDP array indexes by struct UdpHeader ## Description * less error prone * decrease code size to flash (save some bytes, see below) * move `ethernet_header()`, `ethernet_payload()`, `ip_header()`,...

# [IP] use IP header instead of array offsets ## Description * less error prone * decrease code size to flash (save some bytes, see below) * introduce ethernet_header(), ethernet_payload(),...

Add ARP cache

4

Pros: - keep MAC/IP relations into an ARP 'store'. - fix issue when IP packet is received from local network: answer is no more broadcasted - define ETHERCARD_ARP_STORE_SIZE to set...