back.pysim: emit toplevel inputs in VCD files as well.

back.pysim: emit toplevel inputs in VCD files as well.

Before this commit, only signals driven from fragments (in practice,
everything except toplevel inputs) would get written to a VCD file.
Not having toplevel inputs in the dump made debugging ~impossible.

After this commit, all signals the fragment refers to get written to
a VCD file. (More specifically, all signals the compiler assigns
an index to, i.e. signals the generated code reads or writes.)

Fixes #280.