floor and ceil of an infinity or NaN throw OverflowException

[Rafael-SOWNet]

floor and ceil of an infinity or of NaN throw System.OverflowException: Value is infinity or NaN out of evaluation. Introduced by #827.

An internal PeterO.Numbers exception reaching the caller is a bug on its own terms — the library's
contract is that evaluation answers, returns an unevaluated node, or returns NaN, not that it throws
a numeric-library exception the caller has no reason to expect.

Measured on master (46140f8)

eval(floor(+oo))     !!! OverflowException: Value is infinity or NaN
eval(ceil(-oo))      !!! OverflowException: Value is infinity or NaN
simplify(floor(+oo)) !!! OverflowException: Value is infinity or NaN
eval(floor(0/0))     !!! OverflowException: Value is infinity or NaN
eval(floor(x/0))     !!! OverflowException: Value is infinity or NaN

For contrast, the neighbours that behave:

eval(abs(+oo))       +oo
eval(sgn(+oo))       1
eval(abs(0/0))       NaN

Cause

Evaluation.Continuous.Arithmetics.Classes.cs:275-290 (and the Ceilf copy at 299-311):

Real n when !isExact => Integer.Create(n.EDecimal.Floor().ToEInteger()),
Complex n when !isExact => Complex.Create(
    n.RealPart.EDecimal.Floor(), n.ImaginaryPart.EDecimal.Floor()),

Real includes the infinities and NaN. EDecimal.Floor() passes them through, and
EInteger.ToEInteger() refuses them.

What the answers should be

floor(+oo)  +oo         ceil(+oo)  +oo
floor(-oo)  -oo         ceil(-oo)  -oo
floor(NaN)  NaN         ceil(NaN)  NaN

floor of an infinity is that infinity — there is no greatest integer below +oo, and +oo is the
answer every other CAS gives and the one abs already gives here. NaN propagates, as it does
everywhere else.

The complex branch needs the same guard componentwise: floor(1/2 + oo*i) should not throw either.

Related

The limit crash in the same node pair is #829. Both come from #827 and both are cases the node's own
tests did not reach; a checklist of the places a new subtype must be wired would have caught both.