render addresses of non-pointer values that have pointer references

[kortschak]

At the moment spew does not render addresses of non-pointer values that are referenced by pointers in a dump, this is described in more detail in my issue here kortschak/utter#2.

I have an incomplete implementation of this (its working but I'm still thinking about how to deal with []byte dumps), that I'd be happy to donate back to spew: https://github.com/kortschak/utter/commit/b77c4ff3a3901e89d3074ad69c6618948708e016 (minor changes in addition to the changes in this CL are required - basically just the wasPtr return value from unpackValue).