Reduce message overhead

[edsko]

Small CH messages have a large overhead. For instance, consider sending (Nothing :: Maybe ProcessId). The binary encoding of Nothing is one byte, but the total message sent looks like

<channel ID>                4 bytes
<message length>            4 bytes
<message type fingerprint>  16 bytes
<payload>                   1 byte

For an overhead of 2400% :) Obvious ways to reduce the overhead are

• Use a single byte for the first 255 connection IDs
• Use a single byte for message length <= 255
• Use a single byte to index into some cache for commonly used fingerprints (since most applications will probably send only a handful of different types of messages across the network, this might be very effective)