Add support for symbolic watchpoints

[travitch]

Following #38, which involves unconditional breakpoints, it would be really nice to have /conditional watchpoints/ that trigger when a condition is true. Unlike a normal debugger, we have some options.

• We clearly wouldn't want them to trigger if the condition is concretely false
• We almost certainly want them to trigger if the condition is concretely true
• We may want them to trigger when the condition may be true - especially if we can provide a term explaining the conditions under which it might trigger.

As far as UI, this would be much like the breakpoint functionality. Supporting source-level watches would be simple (tied to a source location). Supporting adding located watches from the TUI would involve some more effort:

• We need a method for specifying the condition under which the watch should trigger. There could be a UI for assembling simple predicates, but that isn't very satisfying. Specifying them via a programming language (or perhaps JSON?) would be really nice.
• The performance characteristics are interesting - aside from the concretely true and false cases, determining whether or not the watchpoint should trigger requires a solver invocation, which can be expensive. Communicating that to users would be an interesting challenge.