Complete NotANumber

[PatrickHaecker]

NotANumber existed but could not really be used except as the value itself. Every
operation on it was a MethodError or an ErrorException, so an undefined result
could not be carried any further. Most of the places that should have produced a
NotANumber threw instead.

The current PR makes it behave like a float NaN instead.
It answers isnan, compares false against everything including itself, keeps isequal
and hash so a container can hold one, sorts last, and converts to the NaN of any
float type. Every arithmetic operation propagates it.

It also becomes a Real. Dividing two real numbers has to give a real number, and
+∞/+∞ is a NotANumber. Previously, it had isreal(+∞/+∞) false.
Real as supertype allows Complex{NotANumber}, so an operation with a complex
operand answers NotANumber() + NotANumber()*im, like the floats do.

Behaviour that changes:

• ∞ + (-∞) and 0 * ∞ answer NotANumber() instead of throwing an ArgumentError.
• NaN + ∞ answers NotANumber() instead of the float NaN. Every other float special
  value already loses its precision here — Inf32 + ∞ is ∞.
• isreal(NotANumber()) is true.

mod is deliberately left alone. Base answers mod(-2.0, Inf) with -2.0, not NaN,
so NotANumber() would be the wrong answer there; mod(x, ∞) keeps throwing as before.

Based on #82, so it stays a draft until that is merged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (aabb400) to head (a28f6ba).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #86   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          301       336   +35     
=========================================
+ Hits           301       336   +35     

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[PatrickHaecker]

Test coverage is now 100%.

[PatrickHaecker]

Working at ComplexInfinity it becomes more and more clear that NotANumber should probably be Real to not clash with Base definitions in the complex case. I am currently trying whether that becomes more consistent if we change the current PR accordingly. So let's wait and see.

[PatrickHaecker]

Yes, this is more consistent. So changed accordingly.