Use fix-846 and implement hyper functions for abstracting over sensitivity

[henriman]

At least at the moment, the hyper functions that should be implemented in this PR/branch are IsLow interface functions (and related functions such as AssertIsLow; "interface sensitivity"), as well as functions like IsLowSlice that abstract over the sensitivity of slices ("slice sensitivity").

Due to Gobra issue #955, we can't yet use fix-846 (Gobra branch) to implement IsLow interface functions. (Many TODO comments mentioning Once Gobra issue #846 is resolved are now actually dependent on issue 955.)

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Current status

• (private/topology/underlay)
• private/topology
• pkg/addr
  • Slice sensitivity
    • We want to introduce a specialized IsLowSlice function for HostIPv4. However, this is not possible yet, due to Gobra issue #963. Otherwise, this is done
  • Interface sensitivity
• private/underlay/conn
  • Slice sensitivity
    • We want to introduce IsLowIP, a specialized IsLowSlice function, for UDPAddr. However, this is not possible yet, due to Gobra issue #963. Otherwise, this is done
  • Interface sensitivity
    • UDPAddr (can't express lowness of IP, see above)
    • Conn interface (mark hyper, implement IsLow functions)
    • Otherwise done (including Config, UDPConn)
• pkg/slayers/path/empty
• pkg/slayers/path
• pkg/slayers/path/onehop
  • Interface sensitivity
• pkg/slayers/path/epic
  • Interface sensitivity

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Suggestion: IsLow functions that are already accompanied by a RevealIsLow and/or AssertIsLow function could be made opaque.