This fixes the *Segment.Concat and RConcat methods to handle the new
number types. It also adds more tests of concatenation with the "Text"
and "Segment" types with number types.
Related #1340
Including the float64 data type in the prompt value stream causes an error.
This causes those values to be implicitly converted to a string as happens
everywhere else in Elvish.
I initially intended to modify the code to do the implicit string coercion
for any type not otherwise explicitly handled by the pkg/ui `Concat`
methods. I decided against that approach because doing so doesn't make
sense for some types that might appear in the value stream; e.g., Elvish
exceptions.
Fixes#1186
