statprof: require paths to save or load profile data
Upstream appears to aggressively save statprof data in a well-defined
home directory path. Change the code to not do that.
We also change file saving to fail if an error has occurred
instead of silently failing. Callers can catch the exception.
This behavior is more suitable for a generic "library" module.
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273ce12ad8f1` will report some style violations on a very old commit).