[symboltable] Improve data structures for holding information on module use statements

[arporter]

#293 is adding basic symbol table functionality. However, it does not yet capture all available information on module use statements. In particular, if we have some code that does:

use my_mod
...
use my_mod, only: var

Then we know that all public symbols in my_mod are available and that one of them is named "var". Currently, we don't capture the latter.