Make header field lengths and descriptions consistent

[nicorusti]

We describe header fields and their lengths in various ways, for example:

• FlowID: This 20-bit field
• PayloadLen: Specifies the length of the payload in bytes. The payload includes (SCION) extension headers and the L4 payload. This field is 16 bits long, supporting a maximum payload size of 65'535 bytes.
• Sometimes field length is missing and it is only inferrable from a figure

We should switch to something more consistent.

Ideas:

• FieldName (xx bits): field description
• Define and use a custom format, like RFC9000 (QUIC):

   Example Structure {
     One-bit Field (1),
     7-bit Field with Fixed Value (7) = 61,
     Field with Variable-Length Integer (i),
     Arbitrary-Length Field (..),
     Variable-Length Field (8..24),
     Field With Minimum Length (16..),
     Field With Maximum Length (..128),
     [Optional Field (64)],
     Repeated Field (8) ...,
   }

I'd ask Eliot for recommendations before doing anything, so I keep this issue with low prio