Add a UI for initializing the symbolic execution state

[travitch]

We currently allocate fresh values for every function argument before running the symbolic execution engine. This is a decent start, but we need to provide more user control:

• Users should be able to specify concrete values
• We need the ability to provide symbolic global states, too
  • This includes values for globals
  • It also likely includes heap allocations (reachable from arguments or other globals)

That could get really complicated, so having some automated support for telling users what has to be populated would be really helpful. The easiest shortcut is starting from a saved state template (possibly obtained via #6).