Add --verification-limit to limit the number of files created

[LouisJenkinsCS]

Currently, when running applications which have a ton of bugs, the amount of data can quickly eat up hard disk space, taking up hundreds of gigabytes in minutes, and likely terabytes within an hour. This is due to the vast number of verification data that is produced; it would be great to limit the number of verifications that are kept around; perhaps keeping a FIFO of verification files that can be removed as new ones are generated? Perhaps a way to keep either --keep-first-dumps=N or --keep-last-dumps=N? With the former you get more performance as less verification files are generated; with the latter you get the most recent results, which may be more important than the initial ones; this does require the verification function to be created however.

As well, maybe the heap is not necessary for all of them to keep around; perhaps it is more advantageous to only keep the .stderr, .stdout, and .dump files only as they are significantly smaller in size than the heap itself.

[LouisJenkinsCS]

Moved this back down to low priority as currently the binary heap is only preserved if specifically requested by the user.