Improve handling of bounds in message contexts

[treiher]

The current semantics of the bounds of message context makes using the contexts unnecessary cumbersome. In several use cases, Last must be explicitly set to ensure correct behavior. It should be possible to improve the usability by distinguishing between different kinds of bounds:

• First, Last: Limit usable range of buffer
  • Fix after initialization (especially important when serializing/parsing an inner message)
• Verified_Last (formerly Message_Last): Position of last bit of last verified field
  • Set to First - 1 on context initialization
  • Increased after setting or verifying field
  • Set to First - 1 on reset or write
• Written_Last: Position of last bit containing parsable data
  • Set to First - 1 or Last on context initialization
  • Increased or kept after writing data into context
  • Set to First - 1 on reset
  • Set to Verified_Last when setting field

Invariants

• Verified_Last <= Written_Last
• First - 1 <= Verified_Last <= Last
• First - 1 <= Written_Last <= Last

This is just a design concept. It must be verified whether this concept is suitable for all use cases.