fixes ERA and f32 compilations

[enricozb]

In #125 and #121 I broke compilation of nets with ERA ports and nets with floats respectively. This fixes both.

Specifically,

• when scanning for references in a def, don't consider ERA nodes (which have Ref as a tag)
• when printing ports that are floats, handle NaN, inf and -inf appropriately

I should probably add tests for compiling .hvmc files with floats...

[HigherOrderBot]

Perf run for a5420ad:

file              mode        main          a5420ad1227a
========================================================
sum_tree          intr-singl      10.278 s      10.266 s
                  intr-multi       4.990 s       4.793 s
--------------------------------------------------------
sum_rec           intr-singl      15.393 s      15.020 s
                  intr-multi       7.647 s       7.595 s
--------------------------------------------------------
sum_tail          intr-singl       1.260 s       1.241 s
                  intr-multi       1.234 s       1.185 s
--------------------------------------------------------
c2                intr-singl       0.000 s       0.000 s
                  intr-multi       0.001 s       0.001 s
--------------------------------------------------------
boom              intr-singl       2.198 s       2.242 s
                  intr-multi       2.893 s       2.646 s
--------------------------------------------------------
bitonic_sort_lam  intr-singl      11.886 s      11.460 s
                  intr-multi       5.756 s       5.601 s
--------------------------------------------------------
merge_sort        intr-singl       7.694 s       7.418 s
                  intr-multi       3.818 s       3.743 s
--------------------------------------------------------
radix_sort_lam    intr-singl      10.245 s       9.916 s
                  intr-multi       4.902 s       4.948 s