nat: cleanup, optimize, and improve errors

[thaJeztah]

nat: improve some assertions

nat: add parsePortNumber utility

Add a utility for parsing and validating port-numbers to reduce
repetition and improve errors.

nat: ParsePortRange, ParsePortRangeToInt: reimplement with parsePortNumber

• Split the implementations from the exported ParsePortRange function.
• The implementation returns an int for ports (instead of an uint64),
  which we cast to the right type in ParsePortRange.
• The parsePortNumber gives more informative errors.

nat: NewPort: reimplement with parsePortNumber

nat: ParsePortSpec: reimplement with parsePortNumber

nat: make Port methods more self-contained

• Use strings.Cut directly instead of using SplitProtoPort
• Use parsePortNumber instead of the SplitProtoPort wrapper

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