experiment: make numeric operations selfless, apart from conversions

[crusso]

Build on #412

Make all numeric operations self-less, including

• compare
• equal
  But excluding conversions:
• v.toXXX()
• float.format(...)
  and excluding predicates:
• f.isNaN() etc

Note that Char and and Text are unchanged and still support self.compare(other) and self.equal(other) (as well as compare(self, other)/equal(self, other).

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