Improved memory profiling, new features, bugfixes

Overhauled memory attribution logic:

• uses Python's custom memory management APIs to efficiently disambiguate native vs. Python memory allocations, supplanting the prior approach that employed periodic call stack sampling.
• performs immediate lookup of the location in source code responsible for allocation/deallocation, reducing the "smearing" effect in attributions previously caused by delayed attribution.
• computes average memory consumption (rather than total) for each line of code (using the novel technique of "one-shot" tracing); lines executed many times no longer appear to have consumed large amounts of memory.
• no longer reports negative memory growth from output, caused by lines freeing more than allocating, which has been a source of confusion for some users.
• this release also resolves a memory leak.

Overhauled internal signal handling:

• uses signal actors, an approach based on actors that decouples signal handling logic from the main thread, avoiding the risk of races and deadlocks and simplifying logic

Bug fixes:

• fixed missing handling of pynvml.NVMLError_NotSupported exception (issue #262);
• fixed issue cleaning up after profiling multiprocessor and multithreaded programs;
• fixed issue not accounting for elapsed time when zero frames were recorded (issue #269).

New features:

• added JSON output option (--json);
• added programmatic profile control (scalene_profiler.start() and scalene_profiler.stop()).

Miscellaneous:

• improved documentation.

Note: this release is for MacOS and Linux only.