Support N:1 symbol:address mapping

[surovic]

So if the same address has multiple names, then the second call will declare a new version of the thing. For global variables, I thnk you'd want to use a GlobalAlias, so that all the "public" names can be aliases to an internal private data. For functions, a similar thing could be done, where "public" versions tail-call the internal private version.

Originally posted by @pgoodman in https://github.com/lifting-bits/anvill/pull/45#r494501404