Fixing some language tests by getting `stop` working correctly

This pull request contains a lot of noise because all of the model dumps and compiler tests needed to be updated, thanks to this making a fundamental change to the compiler.  In particular, that change is that the bodies of all procedures, as well as all `ask`/`ask`-like blocks, and also all top-level observer code—all of that stuff now runs its code through `Prims.stoppably`, which takes a thunk as an argument and evaluates it in a `stop`-safe `try`/`catch` block.

Concerns were expressed earlier about how this might negative affect performance.  I don't particularly think that any performance hit we would see would be very avoidable, but here are my benchmarks on the matter:

> Time:    04/07/2014 @ 05:43:13 PM CDT
> Version: 69658cf
> Comment: Without `Procedures.stoppably`
> 
> BZ Benchmark (Nashorn 1.8.0):
> --Average: 61.93 seconds
> --Min:     60.338 seconds
> --Max:     63.196 seconds
> 
> BZ Benchmark (SpiderMonkey C30.0a1):
> --Average: 18.446 seconds
> --Min:     18.324 seconds
> --Max:     18.583 seconds
> 
> BZ Benchmark (V8 Node.js v0.10.26):
> --Average: 29.328 seconds
> --Min:     28.432 seconds
> --Max:     30.061 seconds
> 
> ==========
> 
> Time:    04/07/2014 @ 05:14:26 PM CDT
> Version: 69658cf-dirty
> Comment: Added `Procedures.stoppably`
> 
> BZ Benchmark (Nashorn 1.8.0):
> --Average: 69.009 seconds
> --Min:     67.413 seconds
> --Max:     70.208 seconds
> 
> BZ Benchmark (SpiderMonkey C30.0a1):
> --Average: 21.378 seconds
> --Min:     21.258 seconds
> --Max:     21.448 seconds
> 
> BZ Benchmark (V8 Node.js v0.10.26):
> --Average: 30.007 seconds
> --Min:     29.491 seconds
> --Max:     30.958 seconds

The performance hit in SpiderMonkey and Nashorn is about 15%.  V8 hardly seems to notice that anything happened.

Requesting review by @frankduncan and @SethTisue.