start abstracting out some fqns for looking up debug types.

my current stuff 1) shouldn't be in command.go and 2) needed/needs
extending to handle array types better.
type_signed: generalize a bit.

so that it also works for array types.
switch to using dwarf.Type everywhere

from dwarf.CommonType.  We can always grab the CommonType from the
actual type whenever we need it.  And the 'root' type gives us
more info, so let's not throw that away.