hhvm: add memory leak isolation scripts
This patch adds two shell scripts I wrote based on my experience in T99525 and
T820. The two scripts are designed to help isolate HHVM memory leaks. The
first, hhvm-collect-heaps, enables jemalloc heap profiling and writes a heap
dump every 10 minutes to /tmp/heaps. The second, hhvm-diff-heaps, uses
google-pprof to subtract each diff from its successor, which shows where memory
was being allocated in the intervening time between the two dumps.
I tested both shell scripts and linted them with ShellCheck.