toExternalString for nonreal RR objects

[d-torrance]

The following behavior of toExternalString isn't quite right:

i1 : x = numeric infinity

o1 = infinity

o1 : RR (of precision 53)

i2 : toExternalString oo

o2 = infinity

i3 : value oo

o3 = infinity

o3 : InfiniteNumber

i4 : x = numeric indeterminate

o4 = NotANumber

o4 : RR (of precision 53)

i5 : toExternalString oo

o5 = NotANumber

i6 : value oo

o6 = NotANumber

o6 : Symbol

Ideally, value @@ toExternalString should act like identity, and in both cases, we're ending up with instances of a completely different class!

To solve this, we:

• Introduce NotANumber, a new instance of IndeterminateNumber.
• Introduce the Number_ZZ syntactic sugar for numeric.
• Modify toExternalString to take advantage of these changes.

So now we have:

i1 : toExternalString numeric infinity

o1 = infinity_53

i2 : value oo

o2 = infinity

o2 : RR (of precision 53)

i3 : toExternalString numeric indeterminate

o3 = NotANumber_53

i4 : value oo

o4 = NotANumber

o4 : RR (of precision 53)

This is a draft for now since there are conflicts with #3630 and I should add tests and docs.