[WIP] snapshot feature (issue 535) + many tiny changes

[MetrawareOfficial]

That's FYI. That's WIP. That's big. That's my last day in the company, and the community can benefit.

1. There are ~110 very clean commits of various improvements and fixes.
2. There is Doxygen for its collaboration diagrams: see sObj and WrenCensus.
3. There are ~160 not-so-perfect commits for the snapshot feature.

See dir example/embedding/snapshot See file doc/notes/snapshot.md See function wrenSnapshotWant() for usage.

Existing doc was (proof)read, but doc of snapshot feature is TODO. Swizzle turns out nicely done in layers. All existing tests but 5 pass: APITest_Run() tests more than stdout/stderr. Test runner is improved.

Because stressing GC is useful:

• Issue #654 is fixed.
• Long tests are documented and easily skippable.
• Attributes turn out unsafe: TODO.

The API is bad^Wlegacy. For not learning premake, all code is badly bolted and spread. The restored snapshot is not verified.

[mhermier]

Nice. We will probably have to dissect that patch set in smaller pull requests. Even if everything will not get into wren, I'll definitely archive a copy for myself to work on it.