Rat operations give bogus underflow

[p6rt]

Migrated from rt.perl.org#130155 (status was 'new')

Searchable as RT130155$

[p6rt]

From zefram@fysh.org

2e0**-1019
1.78005908680576e-307
2e0**-1020
8.90029543402881e-308
2.0**-1019
1.78005908680576e-307
2.0**-1020
0
0.5**1019
1.78005908680576e-307
0.5**1020
0
1 / (2.0**1019)
1.78005908680576e-307
1 / (2.0**1020)
0

The three ways of computing 2**-1020 from Rat values all bogusly underflow
to zero. As you can see, the exact value is representable in the Num
type, which all of these Rat operations use for their results.

-zefram

[p6rt]

From @zoffixznet

MoarVM PR fixing this sent​: MoarVM/MoarVM#442