More edge types

[trombonehero]

It would be useful to be able to distinguish among:

• nodes creating nodes
• information flowing explicitly between nodes (read(2), write(2), etc.)
• information possibly flowing between nodes (e.g., mmap(2))
• information flowing implicitly between nodes (e.g., wait(2))

[trombonehero]

This issue will be subsumed by PVMv2 when it lands. There's not really any action to take for now.