Fix STable claiming.

With this, t/serialization/01-basic.t now fully passes.
scsetcode needs to set code object's SC

Fixes remaining failures in t/serialization/02-types.t.
Ensure context_object is NULL and marked.

Avoids one segfault in closure serialization.
Better error reporting.
Pass correct object; fixes 03-closures.t.
Add serialization tests to selftest.
Merge branch 'master' of github.com:MoarVM/MoarVM into gcorch