u3: refactors spin-stack for safety and robustness#1038
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This PR refactors the spin-stack writer (mars, bytecode-interpreter) and reader (urth, http driver) for safety and robustness. Overflow/underflow conditions are checked, inconsistencies in stack-restoration on error are removed, and data races between reader and writer are resolved (by adding atomic-guarded write-operation counters and detecting/retrying writes impacting reads). A memory leak in the http server is plugged, and the relevant fixes from #1029 are recapitulated.
A couple things that are not included but may be worth pursuing: the shared memory namespace still uses PIDs and is therefore liable to collision in certain cloud/containerization scenarios (could be resolved with higher-entropy names), and stack-entry lengths are still 4 bytes (where 2 would suffice).